The Aarne–Thompson tale type index is numerically-coded list of myths, legends, folk tales, songs, and poems, based on motifs and plot features they have in common with each other. Antti Aarne developed and published it in 1910 as Verzeichnis der Märchentypen, intended to be used by folklorists to help them compare, contrast, and keep track of stories. The index was twice revised, expanded, and translated by Stith Thompson, first in 1928 and then in 1961. The tale type numbers themselves were first implemented with Thompson's second revision; up until that point, stories had been categorized under code words and headers, not unlike thesaurus entries. There are approximately 2500 tale types, most of which are applied to folklore of European and Asian origin.
The Aarne-Thompson Index pays less attention to the actions, dialogue, and decisions of characters in any given story; instead, it focuses more on the types, species (such as a wolf, a fairy, or a princess), and specific personality traits (such as foolishness, great age and wisdom, kindness and innocence, or overwhelming good luck) of the characters in the plot. It also focuses on the morphology and mechanisms of the plot which initially set events in motion, such as the presence of an evil giant or ogre, or the jealousy between sisters. The Index uses both broad tale-type generalizations, such as Religious Tale, Fairy Tale, Animal Tale, etc., and very specific subdivisions within those categories, with greater and greater specificity in each nested category.
Within any one tale type, there is considerable room for variation: a tale type involving helpful dwarves might involve the main character's younger brothers, instead of dwarves. A story about a virtuous animal being mistakenly punished after protecting his owner might involve a hunting dog, a hawk, or a mongoose.
In 2004, Hans-Jörg Uther published an expansion and update of the AT Index, in his The Types of International Folktales: A Classification and Bibliography. He remarked on the limitations of the previous system, and he began to integrate Japanese, Chinese, and other East Asian folklores into it, where previously that entire branch of folklore had been unexplored and excluded by the system. There is still some criticism of the AT Index, because it does little to account for tales which have multiple overlapping story types from different categories of the Index, and it also does not provide a basis for contextualizing individual stories within the cultural background where the stories are discovered. Furthermore, simply changing the species of a character (such as re-casting a wolf as the Devil, or a fairy princess as a human princess)would cause that story to be labeled by a completely different AT number, even if all other plot details remained exactly the same. Additionally, the stories and tale types first included in the index came almost exclusively from oral traditions, even though much earlier print forms of those stories already existed; a great many tales were left out of the original index simply because they were not orally transmitted at the time of the first indexing. Many world regions are under-represented by the Index, with Western Europe's folklore making up the bulk of tale types. Despite these criticisms, the Aarne-Thompson Index remains one of the most vital analytical resources for folklorists.
Here are some examples of Aarne-Thompson Indexed story types which will probably be particularly familiar to native English speakers:
Town Mouse and Country Mouse 112
The Three Little Pigs 124
Androcles and the Lion 156
The Three Bears 171
Godfather Death 332
Little Red Riding Hood 333
The Sleeping Beauty 410
Beauty and the Beast 425C
Snow White and Rose Red 426
Tom Thumb 700
The Princess and the Pea 704
Snow White (with or without seven dwarves or brothers) 709
Stone Soup 1548
The Emperor's New Clothes 1620
The Sky Is Falling 2033
This Is the House That Jack Built 2035