There are 88 keys on a full size piano. However there are only 12 unique keys you can play before you begin to repeat keys in another register. That means that you can play most keys up to 7 times over the piano. Some keys you can play up to 8 times (A,Bb, B and C). Once the key is “repeated” in another place it is played in another register an octave (or several octaves) higher/lower.
Octaves 1-7 and the 4 extra keys from the 8th octave on a full size 88 key piano.
In order to find anything on the piano we have to look for certain patterns. Let us look at the black keys on a piano. We can see that there is a group of 2 black keys followed by a group of 3 black keys and then 2 black keys and 3 black keys again and so forth etc. all the way across the piano. We can use these as reference points to finding keys. An octave is the range where all 12 unique keys can be played without repeating any of them. An octave will contain one 2 black key group and one 3 black key group and all the white keys that surround them.
Octave range using the 2+3 black note groups and the white notes surrounding them.
The lowest note on the piano is A and the highest is C. We can use either of them as a reference point to find different things on the piano. C is the key that is most commonly used. It is situated immediately to the left of the 2 black key group. Maybe it is a popular reference point precisely because it marks the start of the 2 black key black note group.
Let us mark C as 1 and then go upwards (to the right) key by key (black and white) until we reach the same key (C) an octave above. We can see that we only have 12 unique notes before they start to repeating themselves.
The 12 unique keys repeating over several octaves.
(c) 2019 Sibil Yanev