The majority of compression fittings out there are 21mm, which, for the most part, means that if you have a tool that works on Digicon fittings (the CP-LCCT1, for example), you can use it on Digicon, F-Conn, Gilbert, PPC EXXL series, PCT DRS series, Holland SuperLOK, and many others. There are others that need a different stroke length, such as the PCT TRS series, or the PPC EX series, which is a different size still. The PPC and the PCT TRS fittings are universal, which means you can use them on standard through quad-shield cable. Digicon and F-Conn make a universal fitting as well.