The IOB format (short for inside, outside, beginning), also commonly referred to as the BIO format, is a common tagging format for tagging tokens in a chunking task in computational linguistics (ex. named-entity recognition). It was presented by Ramshaw and Marcus in their paper "Text Chunking using Transformation-Based Learning", 1995 The I- prefix before a tag indicates that the tag is inside a chunk. An O tag indicates that a token belongs to no chunk. The B- prefix before a tag indicates that the tag is the beginning of a chunk that immediately follows another chunk without O tags between them. It is used only in that case: when a chunk comes after an O tag, the first token of the chunk takes the I- prefix.