Perhaps one of the most unappreciated features of Express Checkout is that it “just works” on most mobile web browsers running a recent version of Android and iOS. When used in this context, you may hear it referred to as “Mobile Express Checkout” (MEC) or “Express Checkout for Mobile.” Tweet Relevance sports a user interface that’s actually optimized for mobile clients, and the decision to implement an Express Checkout turns out to look like an especially smart decision when you realize that literally no additional work is required to deliver an Express Checkout experience that’s optimized for mobile devices. Figure 2-12 shows Tweet Relevance as served up on a mobile device and Figures 2-13 and 2-14 demonstrate the MEC user interface.