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In Music theory, the key is the tonal center of a piece. It is designated by a note name, such as C, and can be either major or minor. What a key designates to a performer is the scale in which all the diatonic notes of the piece lie. This is slightly more complicated in a minor key, because the pitch of the sixth and seventh scale degrees in a minor key can change depending on their harmonic context. The primary key of a piece of music is indicated at the beginning of the piece with a key signature.

A piece may change key at some point. This is sometimes done by just starting in the new key with no preparation - this kind of key change is common in various kinds of popular music, when a sudden change to a key a whole tone higher is a quite frequently heard device at the end of a song. In classical music, however, a "smoother" kind of key change is more usual. A certain number of chords, which may be taken to be in either the old key or the new one, are often used to form a bridge between the two keys. Such chords are sometimes known as "pivot chords". When such a key change also has some structural significance (for example, the key change between subjects in a sonata form movement) it is said to be a modulation.

It is easier to modulate to some keys than others, because certain keys have more notes in common with each other than others, and therefore more possible pivot chords. Modulation to the dominant (a fifth above the original key) or the subdominant (a fourth above) is relatively easy, as are modulations to the relative major of a minor key (for example, from C minor to E flat major) or to the relative minor of a major key (for example, from C major to A minor). These are the most common modulations, although more complex changes are also possible.

Certain musical instruments are sometimes said to play in a certain key, or have their music written in a certain key. Instruments which do not play in the key of C are known as transposing instruments. The most common kind of clarinet, for example, is said to play in the key of B flat. This means that a scale written in C major in sheet music will actually sound as a B flat major scale when played; that is, notes sound a whole tone lower than written. Likewise, the French horn, normally in the key of F, plays notes a major fifth lower than written.

Similarly, some instruments may be said to be built in a certain key. A brass instrument built in, say, B flat, will play a fundamental note of B flat, and will be able to play notes in the harmonic series starting on B flat without using valves, fingerholes, slides or otherwise altering the length of the vibrating column of air. An instrument built in a certain key will often, but not always, have its music written in the same key (see trombone for an exception).

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 In [[Music theory]], the '''key''' is the ''tonal center'' of a piece. It is designated by a note name, such as C, and can be either [[Major scale|major]] or [[Minor scale|minor]]. What a key designates to a performer is the scale in which all the [[Diatonic scale|diatonic]] notes of the piece lie. This is slightly more complicated in a minor key, because the [[pitch]] of the sixth and seventh [[scale degree]]s in a minor key can change depending on their [[Chord|harmonic context]]. The primary key of a piece of music is indicated at the beginning of the piece with a [[key signature]].
-A piece may change key at some point. This is sometimes done by just starting in the new key with no preparation - this kind of key change is common in various kinds of [[popular music]], when a sudden change to a key a [[whole tone]] higher is a quite frequently heard device at the end of a [[song]]. In [[classical music]], however, a "smoother" kind of key change is more usual. A certain number of [[chord]]]]s, which may be taken to be in either the old key or the new one, are often used to form a bridge between the two keys. Such chords are sometimes known as "pivot chords". When such a key change also has some structural significance (for example, the key change between subjects in a [[sonata form]] movement) it is said to be a ''modulation''.
+A piece may change key at some point. This is sometimes done by just starting in the new key with no preparation - this kind of key change is common in various kinds of [[popular music]], when a sudden change to a key a [[whole tone]] higher is a quite frequently heard device at the end of a [[song]]. In [[classical music]], however, a "smoother" kind of key change is more usual. A certain number of [[chord]]s, which may be taken to be in either the old key or the new one, are often used to form a bridge between the two keys. Such chords are sometimes known as "pivot chords". When such a key change also has some structural significance (for example, the key change between subjects in a [[sonata form]] movement) it is said to be a ''modulation''.
 It is easier to modulate to some keys than others, because certain keys have more notes in common with each other than others, and therefore more possible pivot chords. Modulation to the dominant (a fifth above the original key) or the subdominant (a fourth above) is relatively easy, as are modulations to the relative major of a minor key (for example, from C minor to E flat major) or to the relative minor of a major key (for example, from C major to A minor). These are the most common modulations, although more complex changes are also possible.