abstract:In chess, particularly in endgames, a key square (also known as a critical square) is a square such that if a player's king can occupy it, he can force some gain such as the promotion of a pawn or the capture of an opponent's pawn. Key squares are useful mostly in endgames involving only kings and pawns.
The key difference is that Square will allow the customers to pay with credit and debit cards on their phone, while Starbucks's current system only allows consumers to use their Starbucks debit card to pay.