The general objective of this study is a systematic examination of the processes involved in the formation and evolution of complex sentence constructions in a group of genetically related languages. The Chadic language group, at about 140 languages, constitutes the largest and most diversified branch of the Afroasiatic family. One of the findings of the present work is that languages starting from the same base may develop quite different morphological and syntactic structures. With respect to issues of general linguistic interest, the book deals with motivations for grammaticalization: It is proposed that one of the most important motivations is satisfaction of the principle of well formedness, that is, that every element in an utterance must have its role transparent to the hearer either by inherent lexical properties or by grammatical means. In the present work both aspects of grammaticalization, viz. the emergence of grammatical constructions and the emergence of grammatical morphemes, are given equal weight. In addition to semantic metaphor and metonymy as mechanisms in the processes of grammaticalization, the present work develops the notion of semiotic metonymy, whereby a part of a sign performs the function of the sign. It is shown that semiotic metonymy plays an important role in the grammaticalization of grammatical morphemes and constructions into other morphemes and constructions. The book also shows that unindirectionality is not a governing principle with respect to the development of grammatical morphemes into other grammatical morphemes; rather, there is considerable evidence and theoretical justification for the bidirectionality principle.