That is, the value of x is obtained by first applying f to x to obtain y = f(x) and then applying g to y to obtain z = g(y). In the notation gcirc f, the function on the right, f, acts first and the function on the left, g acts second, reversing English reading order. The notation can be memorized by reading the notation as "g of f" or "g after f". The composition gcirc f is only defined when the codomain of f is the domain of g. Assuming that, the composition in the opposite order fcirc g need not be defined. Even if it is, i.e., if the codomain of f is the codomain of g, it is not in general true that