Braces

In generative phonology, 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