In Western music, the adjectives major and minor may describe an interval, chord, scale, or key. A composition, movement, section, or phrase may also be referred to by its key, including whether that key is major or minor. The words derive from Latin words meaning "large" and "small," and were originally applied to the intervals between notes, which may be larger or smaller depending on how many semitones (half-steps) they contain. Chords and scales are described as major or minor when they contain the corresponding intervals, usually major or minor thirds. Intervals A major interval is one semitone larger than a minor interval. The words perfect, diminished, and augmented are also used to describe the quality of an interval. Only the intervals of a second, third, sixth, and seventh (and the compound intervals based on them) may be major or minor (or, rarely, diminished or augmented). Unisons, fourths, fifths, and octaves and their compound interval must be perfect (or, rarely, diminished or augmented). In Western music, a minor chord "sounds darker than a major chord". Scales and chords Major and minor may also refer to scales and chords that contain a major third or a minor third, respectively. A major scale is a scale in which the third scale degree (the mediant) is a major third above the tonic note. In a minor scale, the third degree is a minor third above the tonic. Similarly, in a major triad or major seventh chord, the third is a major third above the chord's root. In a minor triad or minor seventh chord, the third is a minor third above the root. Keys The hallmark that distinguishes major keys from minor is whether the third scale degree is major or minor. Major and minor keys are based on the corresponding scales, and the tonic triad of those keys consist of the corresponding chords; however, a major key can encompass minor chords based on other roots, and vice versa. As musicologist Roger Kamien explains, "the crucial difference is that in the minor scale there is only a half step between '2nd and 3rd note' and between '5th and 6th note' as compared to the major scales where the difference between '3rd and 4th note' and between '7th and 8th note' is [a half step]." This alteration in the third degree "greatly changes" the mood of the music, and "music based on minor scales