C codebases frequently embed nonportable and unstandardized elements such as inline assembly code. Such elements are not well understood, which poses a problem to tool developers who aspire to support C code. This paper investigates the use of x86-64 inline assembly in 1264 C projects from GitHub and combines qualitative and quantitative analyses to answer questions that tool authors may have. We found that 28.1% of the most popular projects contain inline assembly code, although the majority contain only a few fragments with just one or two instructions. The most popular instructions constitute a small subset concerned largely with multicore semantics, performance optimization, and hardware control. Our findings are intended to help developers of C-focused tools, those testing compilers, and language designers seeking to reduce the reliance on inline assembly. They may also aid the design of tools focused on inline assembly itself.