text:  Some of the impedance terms and section terms used in this article are pictured in the diagram below . Image theory defines quantities in terms of an infinite cascade of two @-@ port sections , and in the case of the filters being discussed , an infinite ladder network of L @-@ sections . Here " L " should not be confused with the inductance L – in electronic filter topology , " L " refers to the specific filter shape which resembles inverted letter " L " . 
  