Braces

Braces are a notational convention to abbreviate two rules.

Examples

Braces are a means to abbreviate the rules (i) and (ii) as the one rule (iii).

(i) A -> B / C _____

(ii) A -> B / D _____

C (iii) A -> B / { } ______ D

Link

Utrecht Lexicon of Linguistics

Reference

Chomsky, Noam A. & Halle, Morris. 1968. The sound pattern of English. New York: Harper & Row.