In music theory, the key of a piece is the group of pitches, or scale, that forms the basis of a musical composition in Western classical music, jazz music, art music, and pop music.

Tonality (from "Tonic") or key: Music which uses the notes of a particular scale is said to be "in the key of" that scale or in the tonality of that scale.

A particular key features a tonic (main) note and its corresponding chords, also called a tonic or tonic chord, which provides a subjective sense of arrival and rest. The tonic also has a unique relationship to the other pitches of the same key, their corresponding chords, and pitches and chords outside the key. Notes and chords other than the tonic in a piece create varying degrees of tension, resolved when the tonic note or chord returns.

The key may be in the major mode, minor mode, or one of several other modes. Musicians assume major when this is not specified; for example, "this piece is in C" implies that the key of the piece is C major. Popular songs and classical music from the common practice period are usually in a single key; longer pieces in the classical repertoire may have sections in contrasting keys. Key changes within a section or movement are known as modulation.

Music is made from audible vibrations, such as from oscillating strings or air moving through reeds.. Humans generally perceive some vibrations as sounding "higher" or "lower" than others, creating a subjective perception called pitch. As a medium vibrates faster, its higher frequency is perceived as higher pitch. In Western tonal music, specific frequencies are grouped into twelve pitch classes. Seven of these are considered "natural" pitches, and are represented by the letters A, B, C, D, E, F, and G.

Humans perceive frequencies logarithmically, not linearly. The ratio between two frequencies determines how different they sound. If one vibration has twice the frequency of another, they sound similar enough to be considered the same pitch class; they are said to be one octave apart. Pitches separated by an octave are sometimes given numbers to reduce ambiguity. For example, the lowest C playable on a standard piano is designated C1, while the pitch with twice the frequency (one octave higher) is C2.

On a piano, each white key represents one of the seven "natural" pitches. These are arranged from A to G, followed by another A one octave higher. Among these pitches, the relative increases in frequency from B to C and from E to F are smaller than the changes between other adjacent pitches. This smaller distance is called a semitone, while the larger distance from e.g. A to B is a whole tone. When two natural pitches are a whole tone apart, a black key represents an intermediate pitch. Instead of using separate letters, these pitches are denoted with the accidentals ♯ (sharp) and ♭ (flat), which represent raising or lowering a natural pitch by one semitone. The pitch between G and A may be denoted as either G♯ (G plus one semitone) or A♭ (A minus one semitone). In musical terminology, G♯ and A♭ are considered enharmonic.

Most Western music is built around major and minor scales. These scales use seven of the twelve pitches, arranged in ascending or descending order, and beginning and ending with the same pitch one octave apart. An ascending major scale moves between pitches with the pattern tone, tone, semitone, tone, tone, tone, semitone. An ascending minor scale instead uses the pattern tone, semitone, tone, tone, semitone, tone, tone; this results in the third, sixth, and seventh pitches being one semitone lower compared to the major scale.. On a piano, a major scale beginning from C and a minor scale beginning from A each contain only natural pitches and use exclusively white keys; the two scales are considered distinct "modes" of the same group of pitches. In contrast, a major scale beginning from A uses the pitches A, B, C♯, D, E, F♯, G♯, A. The pitch which begins and ends a given scale is called its tonic.