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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.^[1]

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 → VP

S → NP VP

VP → V NP

NP → 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.

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