A single note means little on its own. Music begins when one note is heard against another, either after it or along with it. Learning to name and hear the gaps between notes is the closest thing music has to learning an alphabet.

Naming intervals

An interval is the distance in pitch between two notes. Intervals are named by counting letter names, and the count includes both ends. From C up to E is a third, because you count C, D, E: three letters. From C to G is a fifth (C, D, E, F, G), and from C up to the next C is an octave, from the Latin for eight. Two notes at exactly the same pitch form a unison.

Intervals come in two kinds of use. A melodic interval is two notes played one after the other, and a harmonic interval is two notes sounded together. The same interval can do either job. A singer leaping up a fifth uses it melodically; a guitarist strumming two strings a fifth apart uses it harmonically.

Quality: perfect, major and minor

Counting letters is not quite enough, because C to E and C to E flat are both thirds, yet they sound very different. So intervals also have a quality. The unison, fourth, fifth and octave are called perfect intervals, a name inherited from medieval theory, which treated them as the purest. Seconds, thirds, sixths and sevenths come in two sizes, major and minor, with minor one half step smaller. A major third spans four half steps, and a minor third spans three. That single half step is the main difference between the bright sound of a major chord and the darker sound of a minor one, as later levels show.

Counting half steps gives every interval a precise size. A perfect fourth is five half steps, a perfect fifth is seven, and an octave is twelve.

Consonance and dissonance

Some intervals seem to settle and others seem to itch. A consonance is an interval that sounds stable and blended; a dissonance is one that sounds tense and seems to want to move. Octaves, fifths and thirds are usually heard as consonant. Seconds and sevenths, which grind against each other, are usually heard as dissonant.

Part of the reason lies in the overtone series from the first level. The octave and fifth blend smoothly because their frequencies form simple ratios, so many of their overtones line up: an octave is two to one, a fifth is three to two. When two notes have overtones that nearly but not quite match, the ear hears a fast wobble called beating, which sounds rough. But physics is not the whole story. What counts as acceptably consonant has shifted over centuries and differs between cultures. Medieval European theorists treated thirds as less stable than fifths; later Western music built nearly everything on them. Some Balkan singing traditions prize the close, buzzing clash of a second. Dissonance is not a flaw. It is the tension that makes the resolution satisfying, much as a question gives meaning to its answer.

The tritone

One interval has a special reputation. The tritone spans six half steps and splits the octave exactly in half, for example C up to F sharp. It is neither a perfect fourth nor a perfect fifth but sits uneasily between them, and it sounds unstable in most Western styles. Later writers nicknamed it diabolus in musica, "the devil in music." Popular books sometimes claim the medieval Church banned it, but historians have found no such ban: theorists simply warned against it in certain melodic situations. Composers have since used its edgy sound on purpose, from sirens and horror films to Leonard Bernstein's love song "Maria" in West Side Story.

Worked example: learning intervals from songs you know

Musicians often learn to recognize intervals by linking each one to the opening of a familiar tune. Try singing these openings and listen to the gap between the first two notes.

The first two syllables of "Somewhere Over the Rainbow" leap up a full octave: the second note feels like the first note reborn higher. "Twinkle, Twinkle, Little Star" jumps from the first "twinkle" to the second by a perfect fifth, open and strong. The bridal march known as "Here Comes the Bride," from Wagner's opera Lohengrin of 1850, begins with a perfect fourth, sturdy but less final than the fifth. The theme from the 1975 film Jaws alternates two notes only a half step apart, a minor second, and that grinding closeness is exactly why it sounds like something approaching. Bernstein's "Maria" opens with the tritone, then quickly slips up a half step to a perfect fifth, as if tension were melting into tenderness.

Notice that none of these require you to see a note on paper. Each is a feeling of distance: wide and calm, open and bold, tight and threatening. Ear training is the slow practice of matching those feelings to names, so that when a teacher says "up a sixth," you can hear it before you sing it.