As teams square off on the opening weekend of the new NFL season, one thing is certain: The players on defense for many teams will include Sam, Mike, and Will. And they might be joined by Jack and Leo.

These aren’t the names of actual players—or if they are, it’s only a coincidence. Rather, it’s the result of a perplexing system of naming defensive positions that has accrued over the years among coaches and players, leaving less-savvy fans scratching their heads. The names are a testament to the game’s constant evolution, and how, as new elaborations on old formations and plays arise, it can require a special lingo to make sense of it all.