The Jacobaeus approximation is more accurate, and to see how it is derived, assume that some particular mapping of calls entering the Clos network (input calls) already exists onto middle stage switches. This reflects the fact that only the "relative" configurations of ingress switch and egress switches is of relevance. There are "i" input calls entering via the same ingress switch as the free input terminal to be connected, and there are "j" calls leaving the Clos network (output calls) via the same egress switch as the free output terminal to be connected. Hence 0 ≤ "i" ≤ "u", and 0 ≤ "j" ≤ "u".