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Two notes form an '''interval'''. This can describe either the ''distance '' between them (, in terms of pitch/semitones(or even further in terms of [[Equal Temperament and Alternative Tunings#Cents|cents]] - a topic discussed later in advanced harmony and tunings) , or the sound produced by playing them together, (simultaneously or in sequence). '''Understanding intervals ''IS'' understanding harmony.''' Scales and chords are composed of a fixed set of intervals that give them their unique sound. Thus, the key of C Major sounds quite like G Major but quite different from C Minor.

You can determine the interval between any two notes by counting the number of notes between themin the common 7-note major or minor scales (or other modes of these scales). For example, F is the 4th of C:

The '''quality''' of the interval will be different depending on which scale you are using and which notes of the scale are being measured. 2nds, 3rds, 6ths, and 7ths will have '''minor''' or '''major''' qualities - minor is closer/smaller and major is further/larger. 4ths and 5ths on the other hand are usually the same distance in these scales, called '''perfect'''; but occasionally, a 5th will be flatted by one semitone, making it a '''diminished''' 5th. Likewise, sometimes the 4th is sharped a semitone, called an '''augmented''' 4th.