In formal language theory, the left corner of a production rule in a context-free grammar is the left-most symbol on the right side of the rule. For example, in the rule A→Xα, X is the left corner. The left corner table associates to a symbol all possible left corners for that symbol, and the left corners of those symbols, etc. Given the grammar S → VPS → NP VPVP → V NPNP → DET N the left corner table is as follows. Left corners are used to add bottom-up filtering to a top-down parser, or top-down filtering to a bottom-up parser.