The diatonic interval in the score above is

A diatonic interval is an interval formed between two notes that both belong to the same diatonic (major or minor) scale. To identify the interval, two things must be determined: the number (how many letter names the interval spans, counting both endpoints) and the quality (major, minor, diminished, or augmented, based on the exact number of semitones).

The score shows two whole notes on a treble clef staff. The lower note sits on the first line of the staff with a flat accidental, making it E♭. The upper note sits in the third space of the staff, which is C.

To find the interval number, count the letter names from E up to C: E, F, G, A, B, C. That is six letter names, so the interval is a 6th.

To determine the quality, count the semitones from E♭ up to C:

E♭ → E → F → F♯ → G → G♯ → A → A♯ → B → C = 9 semitones

A 6th spanning 9 semitones is a major 6th. This can also be confirmed by building the major scale on the lower note: the E♭ major scale is E♭, F, G, A♭, B♭, C, D, E♭. The sixth degree of this scale is C, so E♭ to C is exactly the major 6th of that scale.

The other options can be ruled out as follows:

• A minor 6th contains 8 semitones. For example, E natural to C is a minor 6th, but the flat on E widens the interval by one semitone, making it major instead.
• A diminished 6th contains only 7 semitones, which is narrower still.
• An augmented 6th contains 10 semitones, one more than a major 6th.

When identifying interval quality, remember that flattening the lower note of an interval makes the interval wider (and therefore "larger" in quality), while flattening the upper note makes it narrower. Here, the flat on the lower note E expands what would otherwise be a minor 6th (E to C) into a major 6th (E♭ to C).