This article contains too many or too-lengthy quotations for an encyclopedic entry. (July 2014)
A metronome is a device that produces an audible beat--a click or other sound--at regular intervals that the user can set in beats per minute (BPM). Musicians use the device to practice playing to a regular pulse. Metronomes typically include synchronized visual motion (e.g., swinging pendulum or blinking lights).
A kind of metronome was among the inventions of Andalusian polymath Abbas ibn Firnas (810-887). In 1815 Johann Maelzel patented it as a tool for musicians, under the title "Instrument/Machine for the Improvement of all Musical Performance, called Metronome".
Musicians practice with metronomes to improve their timing, especially the ability to stick to a tempo. Metronome practice helps internalize a clear sense of timing and tempo. Composers often use a metronome as a standard tempo reference--and may play or sing their work to the metronome to derive beats per minute if they want to indicate that in a composition.
When interpreting emotion and other qualities in music, performers seldom play exactly on every beat. Typically, every beat of a musically expressive performance doesn't align exactly with each click of a metronome. This has led some musicians to criticize use of a metronome, because metronome time is different from Musical Time. Some go as far as to suggest that musicians shouldn't use metronomes at all, and have leveled criticism at metronome markings as well. See Criticism of metronome use.
Those in favour of metronome use understand this as a criticism of metronome technique as commonly practiced by musicians, rather than criticism of the tool as such. Their response has been to develop better methods of metronome technique to address the various issues raised by the critics. See Metronome technique. These techniques however aren't widely known by musicians generally, including many critics of metronome use. Metronome technique has developed, but the body of published information is small--so some critics may think that metronome technique consists only of playing music along with the metronome. In his book, Metronome Techniques, Frederick Franz maintains that those who disparage metronomes as "making you mechanical" misunderstand their proper use.
The word metronome first appeared in English c. 1815  and is Greek in origin: metron "measure" and nomos "regulating, law."
Galileo Galilei first studied and discovered concepts involving the pendulum in the late 16th and early 17th centuries. In 1696, Etienne Loulié first successfully used an adjustable pendulum to make the first mechanical metronome--however, his design produced no sound, and did not have an escapement to keep the pendulum in motion. To get the correct pulse with this kind of visual devices, the musician watches the pendulum as if watching a conductor's baton.
The more familiar mechanical musical chronometer was invented by Dietrich Nikolaus Winkel in Amsterdam in 1814. Through questionable practice,Johann Maelzel, incorporating Winkel's ideas, added a scale, called it a metronome and started manufacturing the metronome under his own name in 1816: "Maelzel's Metronome." The original text of Maelzel's patent in England (1815) can be downloaded.
Musicians practice playing to metronomes to develop and maintain a sense of timing and tempo. For example, a musician fighting a tendency to speed up might play a phrase repeatedly while slightly slowing the BPM setting each time. Even pieces that do not require a strictly constant tempo (such as with rubato) sometimes provide a BPM marking to indicate the general tempo.
Tempo is almost always measured in beats per minute (BPM). A metronome's tempo typically is adjustable from 40 to 208 BPM. Another mark that denotes tempo is M.M. (or MM), or Mälzel's Metronome. The notation M.M. is often followed by a note value and a number that indicates the tempo, as in . Specific uses include:
Metronome makers typically mark the speed adjustment for these common tempos:
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A mechanical metronome uses an adjustable weight on the end of an inverted pendulum rod to control tempo. The weight slides up the pendulum rod to decrease tempo, or down to increase tempo. (This mechanism is also called a double-weighted pendulum, because there is a second, fixed weight on the other side of the pendulum pivot, inside the metronome case.) The pendulum swings back and forth in tempo, while a mechanism inside the metronome produces a clicking sound with each oscillation. Mechanical metronomes don't need a battery, but run from a spring-wound clock escapement.
Most modern metronomes are electronic and use a quartz crystal to maintain accuracy, comparable to those used in wristwatches. The simplest electronic metronomes have a dial or buttons to control the tempo; some also produce tuning notes, usually around the range of A440 (440 hertz). Sophisticated metronomes can produce two or more distinct sounds. Tones can differ in pitch, volume, and/or timbre to demarcate downbeats from other beats, as well as compound and complex time signatures. A popular quartz metronome manufacturer is Seiko.
Many electronic musical keyboards have built-in metronome functions.
Software metronomes run either as stand-alone applications on computers and smart phones, or in music sequencing and audio multitrack software packages. In recording studio applications, such as film scoring, a software metronome may provide a click track to synchronize musicians.
Users of iPods and other portable MP3 players can use prerecorded MP3 metronome click tracks, which can use different sounds and samples instead of just the regular metronome beep. Users of smartphones can install a wide range of metronome apps. Either method avoids the need to bring a physical metronome along to lessons or practice sessions.
Perhaps the most famous, and most direct, use of the metronome as an instrument is György Ligeti's 1962 composition, Poème Symphonique for 100 metronomes. Two years earlier, Toshi Ichiyanagi wrote Music for Electric Metronomes. Maurice Ravel used three metronomes at different speeds for the opening of his opera L'heure espagnole (1911).
The clicking sounds of mechanical metronomes have sometimes been used to provide a soft rhythm track without using any percussion. Paul McCartney did this on "Distractions" (Flowers in the Dirt). Following the metronome, McCartney performed a rhythm track by hitting various parts of his body. Also, in Ennio Morricone's theme "Farewell to Cheyenne" (featured on Once Upon a Time in the West), the steady clip-clop beat is provided by the deliberately distorted and slowed-down sound of a mechanical metronome.
In the 20th century the metronome is usually positively viewed by performers, musicologists (who spend considerable time analyzing metronome markings), teachers and conservatories. The common view is reflected in the following quote:
Because its beat is perfectly steady, the metronome is an excellent practice tool for musicians. Practicing with a metronome is extremely useful for developing and maintaining rhythmic precision, for learning to keep consistent tempos, for countering tendencies to slow down or speed up in specific passages, and for developing evenness and accuracy in rapid passages. Most music teachers consider the metronome indispensable, and most professional musicians, in fact, continue to practice with a metronome throughout their careers.-- The NPR Classical Music Companion (2005) 
Often, the metronome by itself may not be enough to learn complex rhythms. However, its importance for all types of practicing and all genres cannot be understated. The infallibility of the machine is a blessing since it removes guesswork; thus, the player can use the metronome to learn to play evenly and to resist the temptation to take extra time when playing a difficult passage. The player must begin with the premise that the metronome is mathematically perfect and categorically correct. From there, s/he must make a personal commitment to play exactly together with this perfect "chamber music partner."-- A Practical Guide To Twentieth-Century Violin Etudes With Performance And Theoretical Analysis. Doctoral thesis (2004) by Aaron M. Farrell
Metronomes are often recommended to students without reservation:
Before a student can be persuaded to use a metronome, he or she has to know why it is important. The most obvious answer is to help keep rhythms even and clean. Another reason is to keep the meter consistent, placing beats in their proper positions in the music. Metronomes can also help a student to find and fix problems. [...] The metronome quickly alerts the player to these problems by suddenly not clicking in time with the player's beats.-- "Make the Metronome Your Friend" by Professor Dr. Steven Mauk
The objection, sometimes heard, that using a metronome tends to make a player mechanical, is not founded on facts. Indeed, the students who play the most artistically are those who have been the most faithful in the use of their metronome when learning their pieces.-- Josephine Menuez, Etude, April, 1932
Numerous other quotations in favour of the metronome, can be found in the book Metronome Techniques: Potpourri of quotations.
The quotations above show the importance of the metronome in the 20th century ("Most music teachers consider the metronome indispensable, and most professional musicians, in fact, continue to practice with a metronome throughout their careers").
The traits that distinguish Modern style [...]: unyielding tempo, literal reading of dotting and other rhythmic details, and dissonances left unstressed. [...]
Modern style [...]: light, impersonal, mechanical, literal, correct, deliberate, consistent, metronomic, and regular. Modernists look for discipline and line, while they disparage Romantic performance for its excessive rubato, its bluster, its self-indulgent posturing, and its sentimentality. Richard Taruskin calls Modernism "refuge in order and precision, hostility to subjectivity, to the vagaries of personality." It is characterized by formal clarity, emotional detachment, order, and precision.-- Bruce Haynes, The end of early music (Oxford University Press)
Modern style [...] It does not usually inflect or shape notes, [...] use agogic accent of placement, add gracing at all generously, or use rubato (tempos are metronomic and unyielding).
Sol Babitz described it as "sewing machine" style, thinking of the rigidly mechanical rhythmic approach, the four equally stressed 16ths, and the limited flexibility in tempo that often characterizes performances of historical repertoire heard in Modern style.-- Bruce Haynes, The end of early music (Oxford University Press)
Modern style is the principal performing protocol presently taught in conservatories all over the world.-- Bruce Haynes, The end of early music (Oxford University Press)
Musicians of a hundred years ago, hearing a cross-section of present-day classical performances, would likely be struck by this primary difference between their performance practice and ours: [...] Our performance practice [...] assumes that a predictably regular beat is conscientiously maintained throughout a movement. [...] We compensate our lack of timing flexibility by a very highly developed sense of tone-color and dynamic which, however refined and polished it may be, tends to abstract and de-personalize the music-making, underscoring its "absoluteness".
The principle of strict unity of beat within a movement has been part of our understanding and experience of classical music for so many decades now, that today's musicians and listeners can hardly imagine that less than a century ago the "standard" classical repertoire was performed under significantly different assumptions.-- Robert Hill, Music and Performance During the Weimar Republic - Chapter 3: "Overcoming Romanticism": On the modernization of twentieth century performance practice
In the early 19th century the metronome was not used for ticking all through a piece, but only to check the tempo and then set it aside. This is in great contrast with many musicians today:
[...] early nineteenth century [...]. There was little interest in using the metronome to tick all the way through a piece of music. But this is how the device is used by conservatory students today.-- Reflections on American music: the twentieth century and the new millennium : a collection of essays presented in honor of the College Music Society by James R. Heintze (Pendragon Press, 2000)
While this section highlights the modern trends of strict mechanical performance as something widespread in the 20th century and now, as early as 1860, some people advocated this type of "modern" performance practice.|Franz Petersilea (ca. 1860) While some in the 19th century welcomed the metronome, others were critical.
A metronome only provides a fixed, rigid, relentless pulse. Therefore, metronome markings on sheet music provide a reference, but cannot accurately communicate the pulse, swing, or groove of music. The pulse is often irregular, e.g., in accelerando, rallentando, or in musical expression as in phrasing (rubato, etc.).
Some argue that a metronomic performance stands in conflict with an expressive culturally-aware performance of music, so that a metronome is in this respect a very limited tool. Even such highly rhythmical musical forms as Samba, if performed in correct cultural style, cannot be captured with the beats of a metronome. A style of performance that is unfailingly regular rhythmically might be criticized as being "metronomic."
... this series of even, perfectly quantized, 16th notes, is no more evocative of samba, than a metronome would be. In fact, this representation neglects what makes up the samba essence in the first place - the swing!-- Understanding the Samba Groove by Pedro Batista
The metronome has no real musical value. I repeat, the metronome has no value whatsoever as an aid to any action or performance that is musical in intention. [...] refer by analogy to the sister art of drawing. Graphic artists understand well enough the essential and generic difference that exists between mechanically-aided drawing on the one hand and freehand on the other. Similarly, musicians ought to distinguish between (1) the sort of timing that results from dull, slavish obedience to the ticking of a soulless machine, and (2) that noble swing and perfect control of pulsation which comes into our playing after years of practice in treating and training the sense of time as a free, creative human faculty.-- The Amateur String Quartet by James Brown III
[...] using the metronome as a constant guide to ramp up the speed or to keep the rhythm. This is one of the worst abuses of the metronome. [...] If over used, it can lead to loss of your internal rhythm, loss of musicality, and bio-physical difficulties from over-exposure to rigid repetition-- Fundamentals of Piano Practice by Chuan C. Chang
A good performance is so full of these minute retardations and accelerations that hardly two measures will occupy exactly the same time. It is notorious that to play with the metronome is to play mechanically - the reason being, of course, that we are then playing by the measure, or rather by the beat, instead of by the phrase. A keen musical instinct revolts at playing even a single measure with the metronome: mathematical exactitude gives us a dead body in place of the living musical organism with its ebb and flow of rhythmical energy. It may therefore be suggested, in conclusion, that the use of the metronome, even to determine the average rate of speed, is dangerous.
What is musical rhythm? Perhaps it is the difference between a performance that is stiff and metronomic in its strict adherence to the beat, and a performance that flows with elasticity and flexibility that emanates from the music itself. A rhythmically musical performance seems to take its cues from stylistic considerations, tempo, phrasing, and harmonic structure, as well as form. Sometimes we may not be exactly sure what makes a piece sound rhythmically musical, but we know it when we hear it.
It should not surprise us that some children do not know instinctively how to play musically. Many youngsters are surrounded by popular music that is rigid and inflexible in its rhythm, characterized by a relentless beat that is often synthesized or computerized. Even some CDs and MIDI disks especially designed for use with piano teaching materials can encourage students to be overly metronomic in their playing. In general, our students may not be familiar with the idea of subtle nuances of tempo, and may need help understanding this.-- Jennifer Merry
Numerous other quotations critical of the metronome can be found at Wikiquote: Metronome.
Metronome technique is extensive and has been the subject of several books. So this short section just summarizes some of the main ideas and approaches. The "intuitive" approach to metronome practise, is to simply play along with a metronome. With metronome technique however, musicians do separate exercises to strengthen and steady their sense of rhythm and tempo, and increase their sensitivity to musical time and precision.
The basic skill required is the ability to play precisely in the pocket with the metronome in a relaxed fashion. This first step helps the musician to relate to the time of the metronome clearly and precisely at the millisecond level, to help internalize a similarly precise sense of time in yourself. It is not a goal in itself, and the aim is not particularly to be able to play like a metronome.
It is harder to play in the pocket with the metronome than one might expect, especially with piano or percussion. That's because the metronome click may seem to vanish when you hit the click exactly - or may be heard less distinctly. The further you are away from the click the more easily you hear the metronome. Musicians who attempt to play in the pocket with a metronome without use of the established techniques for doing this may find that it introduces tension and effort into their instrument technique.
To address these issues, musicians start by learning to play consistently ahead or behind the beat whenever they want to. As a result, they develop a clear sense of "where the click is" and so can also play to hit the click as well, in a relaxed way.
The other thing they do is to listen out to hear how the sound of their playing merges with the metronome to create a new sound when you play precisely in the pocket with the metronome. By listening in this way (and through other exercises) it is possible to play precisely in the pocket with the metronome in a relaxed fashion. At the same time as they work on playing in the pocket, they also work on flexibility and the ability to play in the same precise way anywhere in the beat.
Many exercises are used to help with precision of timing and sensitivity to time, also independence, to make sure you don't become too dependent to the metronome. These exercises include:
And many other exercises. Much of modern metronome technique is to do with various methods to help resolve timing issues, and to encourage and develop a clear sense of musical time and to help with precision of timing.
This steadiness and precision you can develop and encourage through metronome technique does no harm to musical expression in timing and rhythm; indeed one of the motivations is to help with nuances of timing and tempo. An analogy with art may help. It's like Giotto's circle, or Apelles' straight line, if you can play a perfectly steady and precise beat, it helps with nuances of timing., It doesn't mean that you can only play perfectly steady beats, just as Giotto or Apelles impressive displays of technique didn't mean that they could only draw circles and straight lines.
Modern metronome technique addresses the issues of expressive musical rhythms in many ways. For instance, much of the focus of modern metronome technique is on encouraging and developing a good sense of tempo and timing in your playing, and in your mind. So you may work with the metronome in separate exercises to achieve this. When you have a more precise sense of the passage of time, you can then choose for yourself how to use this in your musical performance. You still play in a musically expressive fashion with continually changing tempo and beat; the only difference is that as a result of your work on precision of timing with use of a metronome, you are more aware of what you are doing..
To be an artist one must be able to play in perfect time - slow, fast, or anywhere between. Then one must be able to leave the time at will. This is not the same as having the time leave the player, and that is the effect if one is not able to play with the metronome.
Special metronome exercises are used to help keep this fluid sense of rhythm and timing as you work with the metronome. There are many of them, they include:
At the same time you can work on developing a higher level of awareness of the many natural rhythms in your everyday life and use exercises to help bring those rhythms into your music.
Time Feel, the subject of Chapter 7, is one of the great keys to musicality for rhythm section instruments. But being able to play behind or ahead of the pulse can also add expression to a melodic line. This, along with slight changes in dynamics, creates phrasing in music. The ability to hear the pulse and yet accelerate or decelerate slightly is a great way to incorporate human feeling into a musical performance. Of course, this is all relative to the tempo, and is best achieved relative to a steady tempo. In other words, the more definite your sense of pulse, the better your capability to manipulate it. This also works for the actions of ritardando and accelerando, as they are relative to a steady pulse and are best performed gradually rather than in sudden shifts"
In this way, with suitable metronome techniques, use of a metronome helps you to improve your sense of time and exact timing without causing any of the expected issues for musicality and expressive timing. The thing to bear in mind all the way through is that you use the metronome to help with exact timing - but that the sense of rhythm and musically expressive timing is something that comes from yourself. Rhythm is natural to human beings and pervades our lives, though you may need help to bring that rhythm into music. As Andrew Lewis says in his book:
Rhythm is everywhere. Be sensitive to it, and stay aware of spontaneous occurrences that can spur rhythmic development. Listen all the time and use your imagination. Become a rhythm antenna.
An exact sense of the passage of time doesn't come to humans so naturally (sometimes time may seem to pass quickly and sometimes more slowly) and that's where the metronome can help most. That's how the teachers of metronome technique referenced here think of the tool - as a way to increase your sensitivity to musical time, and develop greater precision of timing and a clearer sense of the passage of musical time - relative to which musicians can then use expressive, natural and fluid rhythms, with as much rubato and tempo variance as they wish for.
If a musician decides not to use a metronome, other methods are required to deal with timing and tempo glitches, and rushing and dragging without its help. These ideas may also be useful as a complementary approach along with metronome technique.
One starting point is to notice that we rely on a sense of rhythm to perform ordinary activities such as walking, running, hammering nails or chopping vegetables. Even speech and thought has a rhythm of sorts. So one way to work on rhythms is to work on bringing these into music, becoming a "rhythm antenna" in Andrew Lewis's words. Until the nineteenth century in Europe, people used to sing as they worked, in time to the rhythms of their work. Musical rhythms were part of daily life, Cecil Sharp collected some of these songs before they were forgotten. For more about this see Work song and Sea shanties. In many parts of the world music is an important part of daily life even today. There are many accounts of people (especially tribal people) who sing frequently and spontaneously in their daily life, as they work, and as they engage in other activities.
"Benny Wenda, a Lani man from the highlands, is a Papuan leader now in exile in the UK, and a singer. There are songs for everything, he says: songs for climbing a mountain, songs for the fireside, songs for gardening. "Since people are interconnected with the land, women will sing to the seed of the sweet potato as they plant it, so the earth will be happy." Meanwhile, men will sing to the soil until it softens enough to dig." 
Musicians may also work on strengthening their sense of pulse using inner sources, such as breath, and subdividing breaths. Or work with the imagination, imagining a pulse. They may also work with their heart beat, and rhythms in their chest muscles in the same way.
Another thing they do is to play music in their mind's ear along with the rhythms of walking or other daily life rhythms. Other techniques include hearing music in ones mind's ear first before playing it. Musicians can deal with timing and tempo glitches by learning to hear a perfect performance in their mind's ear first.
In some styles of music such as early music notes inégales (according to one minority view interpretation) it can be appropriate to use a different approach that doesn't work so much with a sense of inner pulse and instead works on ideas of gestures and is more closely related to rhythms of speech and poetry. Ideas from this approach can be useful for all styles of music.
The basic ideas are:
This technique is especially challenging in its application, because musicians today are so rigidly trained in metrical regularity. Yet, like the beating of the heart, the musical pulse needs to fluctuate in speed as the emotional content of the music fluctuates. Like the natural shifting accents in speech, musical accents need to shift according to the meaning being expressed. To feel perfect, music must be metrically imperfect.
The cognitive partner of hesitation is anticipation: anticipation is created by building up assumption on assumption about what will happen. When the event which should occur fails to happen at the expected time, there exists a moment of disappointment. Disappointment, however, is soon transformed into a rush of pleasure when the anticipated event is consummated. The art is always in the timing.
When the alignment of notes in the score suggests that they be performed strictly and simultaneously, they may be purposely jumbled or played in an irregular or a staggering manner to create a careless (sans souci) effect. This technique gives music a feeling of relaxed effortlessness
This just touches on some of the ideas; for more details, see "The Craft of Musical Communication".
This is a minority view on interpretation of this style of music, but well worth a mention here because of its different approach to musical time and rhythm, and its relevance to the way rhythms can be practised. The more generally accepted view is that Notes inégales were played with the same amount of swing nearly all the time, like modern Jazz.