You may have seen a musical sheet especially the beginner’s guide in reading musical notes. Instead of Do Re Mi, you might have noticed that letters are written. The letters of the alphabet that are used in music are C D E F G A and B. C is equivalent to Do, D is for Re, E is for Mi, and so on.
The reason for this is the keys on the piano do not start on Do but on La which is equivalent to A. Technically, the piano keys start on the letter A to G. Since Do is the start of musical notes which is C, making C the start of the musical letters and not A.