In these situations, it is often useful to know the relationship between, on the one hand, states where ( J 1 ) z , ( J 1 ) 2 , ( J 2 ) z , ( J 2 ) 2 all have definite values, and on the other hand, states where ( J 1 ) 2 , ( J 2 ) 2 , J 2 , J z all have definite values, as the latter four are usually conserved (constants of motion). The procedure to go back and forth between these bases is to use Clebsch–Gordan coefficients.