In music theory, the key of a piece is the tonic note and chord that provides a subjective sense of arrival and rest. Other notes and chords in the piece create varying degrees of tension, resolved when the tonic note or chord returns. The key may be major or minor, although major is assumed in a phrase like "this piece is in C." Popular songs are usually in a key, and so is classical music during the common practice period, about 1650–1900. Longer pieces in the classical repertoire may have sections in contrasting keys.

Methods that establish the key for a particular piece can be complicated to explain, and vary over music history. However, the chords most often used in a piece in a particular key are those that contain the notes in the corresponding scale, and conventional progressions of these chords, particularly cadences, orient the listener around the tonic.

The key signature is not a reliable guide to the key of a written piece. It does not discriminate between a major key and its relative minor; the piece may modulate to a different key; if the modulation is brief, it may not involve a change of key signature, being indicated instead with accidentals. Occasionally, a piece in a mode such as Mixolydian or Dorian is written with a major or minor key signature appropriate to the tonic, and accidentals throughout the piece.

The major scale or Ionian scale is one of the most commonly used musical scales, especially in Western music. It is one of the diatonic scales. Like many musical scales it is made up of seven notes: the eighth duplicates the first at double its frequency so that it is called a higher octave of the same note (from Latin "octavus", the eighth).

The simplest major scale to write is C major, the only major scale not to require sharps or flats:

The major scale had a central importance in European music, particularly in the common practice period and in popular music, owing to the large number of chords that can be formed from it. In Hindustani classical music it is known as Bilaval.

Structure

A major scale is a diatonic scale. The sequence of intervals between the notes of a major scale is:

where "whole" stands for a whole tone (a red u-shaped curve in the figure), and "half" stands for a semitone (a red broken line in the figure).

A major scale may be seen as two identical tetrachords separated by a whole tone. Each tetrachord consists of two whole tones followed by a semitone: