Intervals

An interval is the distance between two notes - the gap you actually hear when one note follows or sits beside another. It's the smallest unit of harmony, and the vocabulary every chord is built from. Name a few intervals by ear and the rest of music theory starts to click into place.

Pick a root, then hold an interval button to hear it built up from that root. Or play any two notes - on the keys, your computer keyboard, or MIDI - and the readout names the interval. Switch on Hear it melodically to sound the two notes one after the other, the way you'd recognise an interval in a tune.

Quality and number

Every interval has two parts to its name: a number (how many letter-steps it spans - a third, a fifth) and a quality (major, minor, perfect…). A major third is four half-steps; a minor third is three. Stack a major third and a minor third and you've built a major chord - which is exactly where the Chords concept begins.

The tritone - one gap, two names

Six half-steps splits the octave exactly in half: the tritone. It's the one interval the page won't pin to a single name, because it's genuinely both an augmented fourth and a diminished fifth - which one you call it depends on the key you're in, and keys are a later concept. Until then, both names are shown and neither is chosen, the same way the black keys keep both of their spellings.

Beyond an octave

Two notes more than an octave apart form a compound interval - an octave plus a simple one. An octave plus a major third is a major tenth. The buttons here cover the simple intervals up to the octave; play wider on the keys and the readout will name the compound. The naming trick is simple: add 7 to the simple number, so a major second becomes a major ninth, a minor sixth a minor thirteenth.

Where the names come from

Why "perfect" rather than major or minor? The perfect intervals - the unison, fourth, fifth and octave - are the most stable and consonant, so close in sound to the home note that they help your ear locate it. They are also the intervals that stay the same in both the major and minor scales, which is what earns them the "perfect" label. The other intervals carry varying amounts of dissonance, a restlessness that wants to resolve back toward a stable note. The number in an interval's name, meanwhile, comes from counting scale degrees - a third spans three letter-names, a fifth spans five.

Inverting an interval

Flip the two notes - move the lower one up an octave so it sits on top - and you get the interval's inversion. The two always add up to nine: a third inverts to a sixth, a second to a seventh, a fourth to a fifth. The quality flips too, major swapping with minor and augmented with diminished, while perfect intervals stay perfect. The tritone is the curiosity: it is its own inversion, six half-steps either way. Naming intervals against a fixed root is exactly how chords and scales are defined.