Now, if we have chosen "a", "b", "u", and "v", then we have "a*" = - "a", "b*" = - "b", "u*" = - "u", and "v*" = - "v". This means that if we assign a given number to a variable, say "a" = 1, then its complement will be assigned to "a*", i.e. "a*" = - 1. Thus out of eight unknown variables, it is sufficient to specify the value of only four variables. We will consider "a", "b", "u", and "v" as independent variables, while "a*", "b*", "u*", and "v*" as dependent variables. This allows us to consider a bone number ± x as a single number regardless of sign because (1) its assignment to a given variable, say "a", will automatically imply that the same number of opposite sign will be shared with its complement "a*", and (2) two independent variables, say "a" and "b", cannot be assigned the same bone number. But how should we choose "a", "b", "u", and "v"? We have the sum of the top row and the sum of the right column as