What do the BILOU tags mean in Named Entity Recognition?

Question

Title pretty much sums up the question. I\'ve noticed that in some papers people have referred to a BILOU encoding scheme for NER as opposed to the typical BIO tagging sche

Answer 1

Based on an issue and a patch in Clear TK, it seems like BILOU stands for "Beginning, Inside and Last tokens of multi-token chunks, Unit-length chunks and Outside" (emphasis added). For instance, the chunking denoted by brackets

(foo foo foo) (bar) no no no (bar bar)

can be encoded with BILOU as

B-foo, I-foo, L-foo, U-bar, O, O, O, B-bar, L-bar