 Several L @-@ shape half @-@ sections may be <unk> to form a composite filter . Like <unk> 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 <unk> <unk> faces another <unk> , the section is called a <unk> section . Where <unk> faces <unk> 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 <unk> elements . 
