The concept of key, of a piece of music being "in" a key, has been one of the fundamental organizing principles of Western music since the seventeenth century. It is a harmonic organizing principle. When a piece is in the key of C major, for example, or "in C major," it means that the harmony of C major serves as the music's home base, its harmonic center of gravity. Harmonies are defined by several notes sounding together, by chords. For a piece in C major, the "home" harmony is defined by a C-major chord, which is in turn based on — or built from — the notes of a C-major scale. Every key is based in this way on a scale — major keys on major scales, minor keys on minor scales. And since there are twelve different major scales (that is, major scales starting on twelve different notes) and twelve different minor scales, there are twelve possible major keys and twelve possible minor keys.

But back to C major. A piece in C major will establish the C-major harmony at the start and return to C major by the end. In between, however, it may include a wide variety of other chords and harmonies, both major and minor. A piece that is in one particular key overall may also spend time in other keys, temporarily shifting harmonic centers in a compositional process called "modulation.". And while the first and last movements of a multimovement piece such as a symphony, sonata, or concerto will be in the home key, the middle movements are usually written in a completely different key.

Another word that is often used for key is tonality, as in, "the tonality of G minor." Tonality also refers to the overall system of writing music in keys, as in, "the principles of tonality." Music that is written in a key is called "tonal music," and in any piece, the harmony of the home key is called the "tonic."

In the world of tonality, or tonal harmony, harmonies don't follow each other randomly. They are ordered in progressions, in which harmonies that contain dissonances — jarring or unsettling sounds — always eventually lead, or "resolve," to harmonies that are made up of consonances — pleasing, comfortable sounds. In tonal harmony, it is dissonance that creates tension and consonance that provides resolution — or repose, or solidity, or affirmation of some kind.

And nothing is more solid or affirmative than the tonic, the harmony of the home key. The crucial idea of tonality remains the idea of gravity toward the tonic: when a piece is written in a key, no matter how far afield the harmonies may wander, ultimately they give a sense of gravitating back, of wanting to come home. As we listen, much of this process of harmonic leading and gravitation operates on an unconscious level: we're not necessarily aware that we're being drawn along, either away from the tonic key at first or back to it later. But the effects are there all the same, and the genius of tonal music is that by virtue of these effects — effects created by the manipulation of consonance and dissonance — harmonic progressions can create a narrative, or an entire dramatic structure complete with direction, conflict, tension, uncertainty, and satisfying resolution.

Why do composers choose certain keys for their pieces? This question has a variety of answers. To a certain extent, composers choose keys the way abstract painters choose colors. Why blue for a particular pattern, or red, or yellow? It just seems right. Sometimes music simply "comes to" a composer in a particular key, and he or she can't imagine it in any other. A strange concept? Perhaps, but many composers have absolute pitch (or "perfect" pitch), and for them all pitches, and therefore all keys, have distinct, individual identities. Indeed, some composers associate certain moods, characters, or colors — actual colors or emotional ones — with certain keys. These associations are not necessarily consistent, however, and they vary greatly from composer to composer. Even the major-minor difference is not absolute: minor keys are generally heard as "darker" than major keys, and more appropriate for expressions of sadness or seriousness, but music in major keys is by no means always "happy" or light. Aesthetic choices are often combined with technical considerations in a composer's key selection: some instruments sound better in certain keys, and fingering patterns on some instruments may be awkward or comfortable depending on the key of the composition. The key affects the range in which an instrument must play, and instruments sound different in different parts of their range, even if not necessarily better or worse. For singers, the key of a piece may determine whether they will be singing in comfort or straining at the edges of their vocal range.

Does it make a difference, then, if a piece written in D major is played in E major? Yes. Even if we don't know that we're listening to it in a different key, and even if we can't quite identify the difference, the piece will sound different. In some cases this isn't so bad — some pieces sound just fine "transposed," that is, shifted to another key. Most of the time, however, and especially with very familiar works, we'll have a sense that something is wrong — or at least not quite right. Would the paintings from Picasso's "blue period" look good in green?

When a piece is described as being "in C," or "in D," for instance, without mention of major or minor, it's the major key that's indicated. "Minuet in G," for example, means "Minuet in G Major." The lowercase letter is often used to indicate minor: "Symphony in f" means "Symphony in F Minor." Music written in a key includes on each printed line of music an indication called a "key signature," which specifies the key for the performer.

It's worth noting that key is by no means a concept confined to classical music. We don't usually hear radio announcers saying things like, "And now Elvis Presley singing 'You Ain't Nothin' but a Hound Dog,' in C major," but the song is in the key of C major, and just about every pop, rock, country, bluegrass, or jazz tune in existence is written in a key.

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The NPR® Classical Music Companion: Terms and Concepts from A to Z by Miles Hoffman, published by Houghton Mifflin Company. Copyright © 1997 by Miles Hoffman and National Public Radio. All rights reserved.