text:  Some of the <unk> 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 <unk> being discussed , an infinite ladder network of L @-@ sections . Here " L " should not be confused with the <unk> L – in electronic filter <unk> , " L " refers to the specific filter shape which resembles inverted letter " L " . 
  