Inside Outside Beginning

The IOB format (short for Inside, Outside, Beginning) is a common tagging format for tagging tokens in a chunking task in computational linguistics (ex. Named Entity Recognition). The B- prefix before a tag indicates that the tag is the beginning of a chunk, and an I- prefix before a tag indicates that the tag is inside a chunk. The B- tag is used only when a tag is followed by a tag of the same type without O tokens between them. An O tag indicates that a token belongs to no chunk.

Another similar format which is widely used is IOB2 format, which is the same as the IOB format with the difference that the B- tag is used in the beginning of every chunk (i.e. all chunks start with the B- tag).