Octave numbers
The keyboard is divided into octaves, and each one has a number: middle C is C4, the octave below it starts on C3, the one above on C5. Each octave runs from C up to B, and every note carries the number of its octave.
This is scientific pitch notation. The number changes at C: B3 is the last note of octave three, and the very next key is C4. So the D just above middle C is D4, and the B just below it is B3. The lowest key of the piano is A0, the highest C8.
The system is used in English-speaking countries, in software and in tuning apps. Lithuanian, Russian and German music schools use the Helmholtz names instead: great octave (C), small octave (c), then the one-line, two-line and three-line octaves (c¹, c², c³), where c¹ is middle C. The letters C, c, c¹, c², c³ match C2, C3, C4, C5, C6 exactly.
Octave numbers matter when you read: a C an octave too high is a different note. Learn where C4 is and count the other C’s from it.
What it sounds like
Each C sounds an octave higher than the last: the same note, only brighter. C2 is a deep bass; C6 is high and clear.
Where it is taught: the studio’s interactive textbook, chapter 16, “Ledger lines and octaves”.