Analysis in the field of lexical semantics, focussing on one of the fundamental operations in set theory: complementation. It is argued that taking the set-operation of complementation as an onomasociological starting-point provides the means for a coherent unifying account of many aspects of the complexity of uses of individual lexical items. Object of study is a group of predominantly `functional' or `grammatical' words of often fuzzy categorical status: lexical items belonging to at least one of thenotional domains of `complementation', `exception' and `restriction', (e.g. `other', `else', `save', `except', `alone', etc.) in English and German.