When do the Manley-Rowe relations really hold?

Difficulties arising because of a standard assumption in the use of the Manley-Rowe relations, that generator frequencies are incommensurable, are discussed from a physical and mathematical viewpoint, and relations are developed which are valid for any generator frequencies, by making realistic assumptions concerning the generators. The use of these relations is shown to be equivalent to the use of the standard relations when the assumption is dropped that filters used in the circuitry have infinitely narrow bandwidths.