Terms:

interval
staff size
quality
natural interval
simple interval
compound interval
harmonic interval
melodic interval
enharmonic intervals

An interval is the musical distance between two pitches. The ability to spell or recognize intervals quickly and accurately is fundamental to other musical skills such as transposition, chord spelling, chord recognition, and melodic or harmonic analysis. You've already learned about the intervals commonly referred to as half- and whole-steps and seen their usefulness in spelling and identifying different types of scales.

An interval is named by its staff size, i.e. the number of lines and spaces it encompasses in staff notation, and more specifically, by its quality, or size in semitones (half steps) relative to other intervals of the same staff size. As an example, let’s consider the interval of an octave. Another name for an octave is “perfect eighth”—“eighth,” the ordinal form of “eight,” is the staff size; “perfect” is the quality. Counting the lines and spaces from note to note in Example 12-1, you can confirm that the octave's staff size is eight.

Example 12-1

What makes the octave's quality “perfect” is that its pitches are 12 semitones apart. Other eighths are named relative to the perfect eighth: the eighth one semitone larger than an octave is called an “augmented eighth;” the eighth one semitone smaller than an octave is called a “diminished eighth.”