Are you a mod or a rocker?

I’ve been working my way slowly through Reingold and Dershowitz’s Calendrical Calculations. This week I hit Chapter 11 on the Mayan and Aztec calendars and came across a notation for modulo arithmetic that wasn’t familiar to me.1 I figured I’d write about it here on the off-chance that any of you would find it interesting. Also, to make it stick in my head a little better.