Left corner

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 a symbol with all possible left corners for that symbol, and the left corners of those symbols, etc.

Given the grammar

Left corners are used to add bottom-up filtering of a top-down parser.

You can use the left corners to do top-down filtering of a bottom-up parser.