In Western tonal music, a key represents the most common pitches and the center of tonal stability in a song or other composition. A key has two components: a tonic pitch and a mode. The tonic pitch is represented by a letter from A through G, sometimes modified by the accidental symbols ♯ (sharp) and ♭ (flat). This tonic represents the musical pitch which a piece will be oriented around and almost always conclude with. The mode may be Major or Minor; if no mode is specified, Major is usually implied. This mode represents a pattern of ascending or descending pitches, which can create a major or minor musical scale beginning with the tonic pitch. Music in a given key will use the pitches from this scale (called diatonic pitches) more often than the pitches outside it (chromatic pitches). Together, these result in keys with names like C Major, A Minor, and B♭ Major. Not all music has a well-defined key. Written music typically begins with a key signature. This tells the performer which pitches are diatonic and should be expected most often. However, the key signature does not specify the tonic pitch. Music can be converted from one key into another by raising or lowering all pitches, which is called transposition. Some instruments, like the clarinet and trumpet, are called transposing instruments because they usually require this; music written for these instruments will be offset from what the performer actually plays. Since the same musical patterns are found across different keys, musical chords are often expressed as numbers based on where their lowest pitches fall in a given scale. This allows common chord patterns to be easily transposed and analyzed independently from a song's key. Relationships between keys can be visualized with a diagram called the Circle of Fifths. This diagram shows which keys share the majority of pitches with each other and which have minimal overlap. It also shows the relationships between major keys and their relative minors (which contains the same pitches but a different tonic pitch), and the pitch differences between keys with the same tonic but different modes. When two keys are closely related, it is easier to transition between them—a process called modulation.
Background Music is made from audible vibrations, such as from oscillating strings or air moving through reeds. People generally perceive some vibrations as sounding "higher" or "lower" than others, creating a subjective perception called pitch. As a medium vibrates faster, this higher frequency of vibration is perceived as higher pitch. In standard Western tonal music, specific frequencies are grouped into twelve pitch classes. Seven of these are considered "natural" pitches and represented by the letters A, B, C, D, E, F, and G. Humans perceive frequencies logarithmically, not linearly. In other words, the ratio between two frequencies, rather than the absolute difference between them, determines how different they sound. If one vibration has twice the frequency of another, both sound similar and are considered the same pitch class. The distance between the two is called an octave. For clarity, these pitches are sometimes labeled with numbers. For example, the lowest C playable on a standard piano is 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 (e.g. from A to B) is a whole tone. When two natural pitches are a whole tone apart, an intermediate pitch is placed between them—on a piano, these are represented by the smaller black keys. Instead of using separate letters, these intermediate 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). These two ways of representing the same sound are called enharmonically equivalent. 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, which results in the third, sixth, and seventh pitches being one semitone lower compared to the major scale. A major scale beginning from C and a minor scale beginning from A each contain only natural pitches, and on a piano will use exclusively white keys; the two scales are considered distinct "modes" of the same set of pitches. In contrast, a major scale beginning from A uses the pitches A, B, C♯, D, E, F♯, G♯, A. The pitch on which a given scale begins and ends is called its tonic.
Definition A key is a way to classify a piece of music as being oriented around a Major or Minor scale. The name of a key, such as A Major, contains both a Tonic pitch and a Mode (either Major or Minor). Since there are 12 pitches which may be used as a tonic, there are 24 distinct keys. If the mode is not specified, a generic term like "the key of C" generally implies a major key. If a piece of music is in a key, it will mostly use pitches from the corresponding scale. The seven pitches that belong to the scale are called diatonic pitches, while the remaining five that do not are called chromatic pitches. A key does not prohibit the use of chromatic pitches, but it does imply they will be rarer than diatonic pitches. The key's tonic represents a pitch of "stability" for the music to orient around. In particular, a composition almost always ends on the tonic. Not all music has a well defined key. Some classical compositions, such as Chopin's Prelude op. 28 no. 2 and Wagner's Tristan und Isolde, do not clearly follow these patterns. Modern atonal compositions, such as the works of Arnold Schoenberg, have no tonal center of stability and freely interweave all twelve pitches.
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