A screen set is the file (and associated graphics) that makes up the GUI (graphical user interface). In other words, the set of files that comprise what is seen on the screen. A screen set can change not just the appearance of Mach3/Mach4 but also the functionality. This means that a function or operation that works in one screen set may not work when a different screen set is loaded. It is suggested to use the default screen set until an understanding of Mach3/Mach4 is achieved. Screen sets are saved in your Mach3 or Mach4 directory as .set files. They are not interchangeable (you cannot load a Mach3 screen in Mach4 or vice versa), however.