While OpenGL is mostly low-level, it did include some higher-level concepts that were only really used on SGI systems. This led to another series of APIs that removed these features to make it easier to implement on common hardware. The best known of these is MiniGL, which is not a separate API, but simply a list of those functions in OpenGL that are guaranteed to be supported across all hardware, thus ensuring that a program limiting itself to those calls will run with maximum performance.