“If less than 5 innings (or 4½ innings if the Cubs are ahead) are played on this date, this coupon may be redeemed for cash, or exchanged for any ticket of this same price, for a regular season game thereafter, if available. No exchanges or refunds except as above.” — Wording on Cubs rain check from the late 1960s
This language has been on Cubs tickets, and baseball tickets in general, for decades, going back nearly to the beginnings of the game. The five-inning (or 4½ innings, if the home team is ahead) has become a standard that all baseball fans understand.