AbstractThe paper deals with the design of 3‐ports consisting of the parallel connection, at port 1, of a lowpass ladder of degreenhaving all its attenuation poles at infinity, with a highpass ladder of degreemhaving all its attenuation poles at DC, the purpose of the design being to achieve a Butterworth (maximally flat) behaviour, simultaneously in the lowpass direction (1‐2) at DC and in the highpass direction (1‐3) at infinity. Norton1has solved the casem = nfor constant impedance filter pairs. the much more difficult case where one of the above restrictions is not met is discussed in general terms and some new results are obtained. Although the problem is far from being completely solved, useful explicit design relations are established for a few filter pairs of low de