Enfilades are a class of tree data structures invented by computer scientist Ted Nelson and used in Project Xanadu "Green" designs of the 1970s and 1980s.
Enfilades allow quick editing, versioning, retrieval and inter-comparison operations in a large, cross-linked hypertext database.
Edit operations in enfilades are performed by "cutting," or splitting the tree down relevant access paths, inserting, deleting or rearranging subtrees, and splicing the pieces back together.
One-dimensional enfilades are intermediate between arrays' direct addressability and linked lists' ease of insertion, deletion and rearrangement.
Later Xanadu designs are more indirect: a growing pool of sharable content pieces, called the istream (invariant stream) is organized into the documents, links and versions—all with virtual addresses—that the users see and work on.
Storing documents using shared content pieces that remember their identities and can trace back to all their appearances, is called Transclusion.
The POOM starts out an identity matrix, then each edit to the document slices and rearranges horizontal strips of the map.
The POOM can be queried in the V->I or I->V directions by searching in squat, wide address ranges or tall, narrow ones.
Enfilades (internal data structures) and istream addresses are not exposed to Xanadu's external interfaces.
Enfilades were trade-secret information until the Xanadu code was made open-source in 1999, and were mentioned but not explained in some publications before that point.
In 1988, the xu88 system utilized the concept of "General Enfilade Theory" of Mark S. Miller, Stuart Greene and Roger Gregory, described as "generating data management trees with an upwardly propagating search property and simultaneously a downwardly imposable structural property".