Theory & harmony
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.
Related terms
Every term in this glossary is taught in context somewhere in Arco's curricula, on the staff, on the keyboard, and on your own fingerboard at the same time.