The social sharing and news aggregation site Reddit provides a unique example of an ecosystem of community-created rules. Not only do individual subreddits create and enforce their own regulations, but site-wide guidelines and norms may also influence behavior. This paper reports on a mixed-methods study of 100,000 subreddits and their rules. Our findings characterize the types of rules across Reddit, the frequency of rules at scale, and patterns of rules based on subreddit characteristics. We find that rules appear to be context-dependent for individual subreddits but also share common characteristics across the site. Taken together, our findings provide a rich description of this ecosystem of rules, motivating further inquiry into underlying mechanisms for rule formation and enforcement in online communities.