An interval is a ruler musicians use to measure pitch space. Intervals can be melodic (notes played in sequence) or harmonic (played simultaneously).
Each interval has its own character: the octave — stability; the fifth — openness; the third — warmth; the second — tension; the seventh — unease. Solfège begins with learning to recognize intervals by ear — the foundation of musical thinking.
Intervals are divided into consonances (the perfect unison, octave, fifth, and fourth; thirds and sixths) and dissonances (seconds, sevenths, the tritone), though the perception of their “sharpness” has shifted through history. By size they are classed as perfect, major, minor, augmented, and diminished, and those wider than an octave are called compound (the ninth, the tenth). A trained “interval ear” lets a musician read and notate music in the mind.
