Texas Instruments TI-83 PLUS - Manual (Page 466)

TI-83 Plus Applications 463
P/Y and C/Y
P/Y is the number of payment periods per year in a financial transaction.
C/Y is the number of compounding periods per year in the same
transaction.
When you store a value to P/Y , the value for C/Y automatically changes
to the same value. To store a unique value to C/Y , you must store the
value to C/Y after you have stored a value to P/Y .