An interval is the distance between two notes, measured in half steps (the smallest step on a piano — one key to the next, including black keys).

Every interval has a number (how many letter names apart) and a quality (perfect, major, minor, or tritone).

Name	Abbreviation	Half Steps	Example from C
Unison	P1	0	C to C
Minor 2nd	m2	1	C to Db
Major 2nd	M2	2	C to D
Minor 3rd	m3	3	C to Eb
Major 3rd	M3	4	C to E
Perfect 4th	P4	5	C to F
Tritone	TT	6	C to F#/Gb
Perfect 5th	P5	7	C to G
Minor 6th	m6	8	C to Ab
Major 6th	M6	9	C to A
Minor 7th	m7	10	C to Bb
Major 7th	M7	11	C to B
Octave	P8	12	C to C

Here's every interval from C, so you can see how they look on the staff:

1. Count the lines and spaces from the bottom note to the top note (including both notes). This gives you the interval number.
2. Count the half steps to determine the quality (major, minor, or perfect).

Or, if you're working in a key:

• Notes that are in the major scale of the lower note produce major or perfect intervals.
• Lowering the upper note by one half step from a major interval gives you a minor interval.

• Perfect intervals: Unison (P1), 4th (P4), 5th (P5), Octave (P8). These sound very stable and consonant.
• Major/minor intervals: 2nds, 3rds, 6ths, 7ths. Major is one half step wider than minor.
• Tritone: Exactly 6 half steps — right in the middle of the octave. Neither major/minor nor perfect. Sounds tense and unstable.

• Half steps between E–F and B–C. No black key between them. This is why the intervals aren't evenly spaced.
• Perfect intervals don't have major/minor versions. They're just perfect (or augmented/diminished, but that's advanced).
• A major interval shrunk by one half step is minor. A minor interval shrunk by one more is diminished.
• Inversions add up to 9. Flip an interval (move the bottom note up an octave) and the numbers add to 9: a 3rd inverts to a 6th, a 2nd inverts to a 7th.