If each delay element of a linear phase low-pass digital filter is replaced byMdelay elements, an(M + 1)-band filter is produced. The transition-width of this(M + 1)-band filter is1/Mthat of the prototype low-pass filter. A complementary filter can be obtained by subtracting the output of the(M + 1)-band filter from a suitably delayed version of the input. The complementary filter is an(M + 1)-band filter whose passbands and stopbands are the stopbands and passbands, respectively, of the original(M + 1)-band filter. If the frequency responses of the original( M + 1)-band filter and its complementary filter are properly masked and recombined, narrow transition-band filter can be obtained. This technique can be used to design sharp low-pass, high-pass, bandpass, and bandstop filters with arbitrary passband bandwidth.