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An interval is the distance in pitch between two notes. The smallest interval in Western music is the half step (semitone), the distance between any two adjacent keys on the piano including black keys. Two half steps equal one whole step (whole tone). Intervals are also named by the number of letter names they span: a second spans two letters (e.g., C to D), a third spans three (C to E), and so on up to an octave. Interval numbers and half-step counts together fully describe any interval.
Count half steps on a piano keyboard, then name the interval number by counting letter names. Associate common intervals with the opening notes of familiar melodies (e.g., a perfect fifth sounds like the Star Wars theme).
An interval measures the distance in pitch between two notes. In Western music, the smallest interval is the half step (also called a semitone), which is the distance between any two adjacent keys on a piano — white or black. This is the fundamental unit of pitch distance, and everything else is counted in multiples of half steps. Two half steps equal one whole step (whole tone). Most adjacent white keys on the piano are a whole step apart, but E–F and B–C are exceptions: those pairs have no black key between them and are only a half step apart.
Interval numbers are a separate naming system based on letter names rather than half-step counts. To find the interval number, count both the starting and ending letter names: C to E spans C (1), D (2), E (3) — a third. This inclusive counting trips up many beginners, who count only the letters in between and arrive at "second." The rule is always count both endpoints. This is analogous to counting floors in a building: from floor 1 to floor 3 is a span of 3 floors, not 2.
The combination of interval number and quality (which you will learn in the next topic) fully describes any interval. For now, the interval number alone tells you how many letter names are spanned: a second spans two letters, a third spans three, a fifth spans five, and an octave spans eight (the same letter name, one register higher). Recognizing these numbers by ear and by sight is the foundation for understanding scales, chords, and harmony.
A useful habit is to associate intervals with the opening notes of familiar melodies. A minor second sounds like the Jaws theme. A major third opens "When the Saints Go Marching In." A perfect fifth opens the Star Wars fanfare. These auditory anchors let you recognize intervals by ear before you can identify them analytically — and that aural recognition is ultimately the goal of all interval training.
Remember that you learned accidentals and enharmonics as prerequisites: a sharp raises a note by one half step, a flat lowers it by one half step. This means the same physical key on the piano can have two different letter names (C# and Db), which affects interval number even when the pitch sounds identical. C to E♭ is a third (C, D, E♭ — three letters), while C to D# is a second (C, D# — two letters), even though E♭ and D# are the same key. Both half-step content and letter-name counting matter.
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