It is useful to observe that any phase factor for a given (j i, m i) pair can be reduced to the canonical form: ( − − 1 ) a j i + b ( j i − − m i ) where a ∈ {0, 1, 2, 3} and b ∈ {0, 1} (other conventions are possible too). Converting phase factors into this form makes it easy to tell whether two phase factors are equivalent. (Note that this form is only locally canonical: it fails to take into account the rules that govern combinations of (j i, m i) pairs such as the one described in the next paragraph.)