Major League Soccer will allow a fourth substitute during MLS Cup playoff matches in 2017 that require extra time.
The International Football Association Board, which oversees changes to the Laws of the Game, has approved experiments for a fourth substitute in extra time.
The fourth sub may be used regardless of the number of subs a team made during regulation.
MLS Cup playoff series (and the MLS Cup final) that are tied after regulation move to two 15-minute extra time periods. If the tie is not broken, the match is decided by penalty kicks.