Quads

Four of a kind — all four cards of one rank, such as four nines. It beats a full house and loses only to a straight flush or a royal flush.

Rare enough that most players remember each one. Making four of a kind by the river in Texas Hold'em happens in roughly one hand in four thousand, which is why a card room can afford to build a promotion around it.

Because the typical qualifying condition is quads or better being beaten — a combination rare enough to keep the jackpot growing but frequent enough across all the tables in a busy room to pay out now and then. Most rules also require both players to use both hole cards.