Tagdal (located in the modern-day Republic of Niger) is a Northern Songhay language, a mixture of Songhay and Berber. In this paper, the author begins by describing Tagdal's inflectional sub-system, which is of Songhay origin. Then he describes its derivational sub-system, of Berber origin. With this data, the author demonstrates that Tagdal speakers were at one time bilingual in both Berber and Songhay, and that Tagdal is a mixed language—a type of contact language that has its genesis in bilingualism and code-switching, rather than in imperfect learning of a superstrate language. The author presents the sociolinguistic situation which existed at the time that Tagdal was in its formative period, and posits some reasons why ethnically-Berber Tagdal speakers may have felt motivated to shift from Berber to Songhay. Finally, he concludes that Tagdal is the result of an abrupt, intentional language shift. Some concluding remarks make generalizations about mixed languages in general, and specifically about mixed languages that came about by abrupt genesis.