Koz is the Turkish word for trump, the suit that beats every other suit in Batak for the duration of a hand. The player who wins the bidding chooses which suit becomes koz. Once set, any trump card outranks all plain-suit cards, so a small trump can capture a trick full of high cards from other suits, provided you were void in the led suit and allowed to trump. Because trumps are so powerful, holding many of them, and knowing how many are still unplayed, is central to winning. The over-trump rule also requires you to beat an existing trump when you can, keeping trump play sharp.