An interval is the distance between two pitches. Intervals are the building blocks of a melody and harmony. In Modules 1 and 4 we discussed three intervals: the octave, whole step and half step. Intervals may be expressed as acoustic ratios of frequencies (see Appendix 2 on the website). We will study intervals expressed as the distance between pitches on the staff and the keyboard; it is also important to sing and listen to them.