The Aarne–Thompson classification systems are two indexes used to classify folktales: the Aarne–Thompson Motif-Index (catalogued by alphabetical letters followed by numerals) and the Aarne–Thompson Tale Type Index (cataloged by AT or AaTh numbers). The indexes are used in folkloristics to organize, classify, and analyze folklore narratives. The indexes are an essential tool for folklorists because, as Alan Dundes explains, "the identification of folk narratives through motif and/or tale type numbers has become an international sine qua non among bona fide folklorists". This system was expanded in 2004 into the Aarne–Thompson–Uther classification system (cataloged by ATU numbers).