Although social conventions are a powerful guide for behavior, the way they emerge in communities is not well understood. We focus on competing conventions for attributing reposts to the original source on Twitter. We analyze a near-complete history of tweets, and observe how the conventions emerged and spread through the network of Twitter users. Initially the most successful conventions were borrowed from natural language ('via' and 'retweeting'), but over time a community-specific convention came to dominate ('RT'). Additional evidence suggests this specificity to the community, along with efficiency of communication and the timing of the adoption, are key variables in the acceptance of the convention.