An interval is the distance between two musical notes. All melodies and chords are made up of a series of intervals. Intervals are measured in numbers, and are calculated by counting the number of letter names (A B C D E F G A) between and including the notes being measured. Within an octave, intervals are: Unison (two notes of the same pitch played or sung together or consecutively), 2nd, 3rd, 4th, 5th, 6th, 7th and octave (two notes an octave apart). Thus A to B is a 2nd interval, as is B to C, C to D etc. A to C is a 3rd interval, A to D is a 4th, A to E is a 5th, A to F is a 6th, A to G is a 7th and A to the next A is an octave.
Intervals may be melodic (two notes played consecutively) or harmonic (two notes played at the same time). Hence two people singing at the same time are said to be singing in harmony.