When Are You Playing Something Else?

Here’s a question that’s occurred to me quite a lot over the past couple of weeks, since Gen Con. I offer it up without much preamble, save for this: Whether you’re hacking a tightly focused game like Apocalypse World or devising a campaign for something broader, like Dungeons & Dragons, you’re almost always moving around a game’s central axis. Maybe you’re in a tight orbit. Maybe you’re drifting away. But at some distance from that gravitational center, you presumably break free and enter either the void between games or the gravity well of some other shining touchstone. So:

When are you no longer playing the game you purport to be playing? When are you so far removed from your game of origin that you no longer tell people that it is what you are playing?