The sections of the hypothetical infinite filter are made of series elements having impedance 2Z and shunt elements with admittance 2Y . 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 impedance 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 impedance on both ports due to symmetry . This image impedance is designated ZiT due to the " T " topology of a mid @-@ series section . Likewise , the image impedance of a mid @-@ shunt section is designated ZiΠ due to the " Π " topology . Half of such a " T " or " Π " 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 impedance of the half @-@ section is dissimilar on the input and output ports : on the side presenting the series element it is equal to the mid @-@ series ZiT , but on the side presenting the shunt element it is equal to the mid @-@ shunt ZiΠ . There are thus two variant ways of using a half @-@ section .