# Interval

Source: https://arcosounds.com/glossary/interval

**An interval is the distance between two pitches, named by the number of staff steps it spans and qualified as perfect, major, minor, augmented, or diminished.**

The number comes from counting inclusively: C up to E spans C, D, E, so it is a third. The quality comes from the exact number of half steps: four half steps make it a major third, three a minor third. Unisons, fourths, fifths, and octaves take the qualities perfect, augmented, and diminished; seconds, thirds, sixths, and sevenths take major, minor, augmented, and diminished.

Intervals are the units everything else is built from. A triad is two stacked thirds, a scale is a specific sequence of seconds, and a key signature exists to keep those sequences consistent. For a string player they are also physical: a major third in first position is a specific finger distance, which is why learning intervals on the fingerboard alongside the staff makes them stick.

Canonical source: https://arcosounds.com/glossary/interval