Several L @-@ shape half @-@ sections may be cascaded to form a composite filter . Like impedance must always face like in these combinations . There are therefore two circuits that can be formed with two identical L @-@ shaped half @-@ sections . Where a port of image impedance ZiT faces another ZiT , the section is called a Π section . Where ZiΠ faces ZiΠ the section so formed is a T section . Further additions of half @-@ sections to either of these section forms a ladder network which may start and end with series or shunt elements .