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Intervals are named by counting the letter names from the lower note to the upper note (inclusive). For example, C to E is a third (C-D-E = three letters), while C to G is a fifth (C-D-E-F-G = five letters). This counting method provides the generic interval name without regard to accidentals, which affect quality but not the basic interval type.
You know note names and octaves, so you can already identify every letter on the staff — A through G, repeating. Interval naming builds directly on that knowledge: an interval is simply the distance between two notes, and you name that distance by counting letter names. The crucial rule is that you count inclusively — you include both the starting note and the ending note in your count. C to E: C (one), D (two), E (three). That is a third. C to G: C, D, E, F, G — five letters, so a fifth. C back to C (one octave up): C, D, E, F, G, A, B, C — eight letters, so an octave (from the Latin for "eight").
The most common mistake is forgetting to count the starting note, which produces an answer that is always one too small — what should be a third gets called a second, what should be a fifth gets called a fourth. Think of it like counting floors in a building: if you start on the first floor and go up to the third floor, you count 1-2-3, not 0-1-2. Both the ground you start from and the destination count.
Notice that accidentals — sharps, flats, naturals — do not change the generic interval name. C to E is a third, and so is C to E♭, and so is C♯ to E♯. All three pairs span the same three letter names. What changes with accidentals is the interval's quality — whether it is major, minor, perfect, augmented, or diminished — which you will learn in the next topic. For now, the letter-counting method gives you the generic interval: the number-name that tells you how many letter-names the interval spans, independent of the exact number of semitones.
This generic name is surprisingly informative on its own. Thirds are the building blocks of chords. Fifths define the basic shape of a power chord and the framework of Western harmony. Octaves identify notes with the same pitch class. Before you can analyze any of those structures, you need to be able to read off the interval number reliably and instantly. Practice by picking any two notes and counting letter names — always inclusive, always starting from the lower note — until the count becomes automatic.
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