# Left corner

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{{Short description|Part of a production rule in a context-free grammar}}
{{one source |date=April 2024}}
In [formal language theory](/source/formal_language_theory), the '''left corner''' of a [production rule](/source/Production_(computer_science)) in a [context-free grammar](/source/context-free_grammar) is the left-most symbol on the right side of the rule.<ref>[http://cs.union.edu/~striegnk/courses/nlp-with-prolog/html/node55.html 9.3 Using Left-corner Tables], Patrick Blackburn and Kristina Striegnitz, Natural Language Processing Techniques in Prolog</ref>

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.

{|
!Symbol
!Left corner(s)
|-
|S
|VP, NP, V, DET
|-
|NP
|DET
|-
|VP
|V
|}

Left corners are used to add bottom-up filtering to a [top-down parser](/source/top-down_parser), or top-down filtering to a [bottom-up parser](/source/bottom-up_parser).

==References==
{{reflist}}

Category:Parsing

{{Comp-sci-stub}}

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Adapted from the Wikipedia article [Left corner](https://en.wikipedia.org/wiki/Left_corner) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Left_corner?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.