 The sections of the hypothetical infinite filter are made of series elements having <unk> <unk> and <unk> elements with <unk> <unk> . The factor of two is introduced for mathematical convenience , since it is usual to work in terms of half @-@ sections where it disappears . The image <unk> of the input and output port of a section will generally not be the same . However , for a mid @-@ series section ( that is , a section from halfway through a series element to halfway through the next series element ) will have the same image <unk> on both ports due to symmetry . This image <unk> is designated <unk> due to the " T " <unk> of a mid @-@ series section . Likewise , the image <unk> of a mid @-@ <unk> section is designated <unk> due to the " <unk> " <unk> . Half of such a " T " or " <unk> " section is called a half @-@ section , which is also an L @-@ section but with half the element values of the full L @-@ section . The image <unk> of the half @-@ section is <unk> on the input and output ports : on the side presenting the series element it is equal to the mid @-@ series <unk> , but on the side presenting the <unk> element it is equal to the mid @-@ <unk> <unk> . There are thus two variant ways of using a half @-@ section . 
