If you’re a fan of How I Met Your Mother , you know that some bets are about money, some are about pride, and one—the —is about the pure, unadulterated terror of an impending open-palmed strike. What started as a simple wager between Marshall Eriksen and Barney Stinson evolved into one of the most iconic and longest-running gags in television history.

Marshall was given a choice: 10 slaps immediately, or 5 slaps to be delivered at any time in the future. Barney, in his infinite overconfidence, chose the 5 slaps, unknowingly signing up for years of psychological warfare. A History of Violence: The Slap Timeline

The Slap Bet was born in Season 2, Episode 9, titled "Slap Bet". The stakes were simple: Marshall bet that Robin was hiding a secret about being a porn star, while Barney bet she was hiding a secret marriage. The prize? One slap, as hard as possible, to the face of the loser.

The Legendary Slap Bet: A Guide to the Ultimate Sitcom Stakes

You can't have a civilized slap bet without oversight. As Commissioner, Lily’s job was to: Rule on who actually won the wager.