Principal Values
The last topic to discuss in this section is called principal values. If you look at only a portion of the sine function, namely where the domain is equal to 
and the range is equal to -1 through 1, then that defines a new specific function y = Sin(x). Notice that the notation ‘Sin’ is now capitalized. The values given in the domain of Sine are called principal values.
The definitions for other specific functions with principal values are:
y = Cos(x) iff y = cos(x) and 
y = Tan(x) iff y = tan(x) and 
(“iff” is the mathematical notation for “if and only if”)