I don't know for sure how many different types of air line fittings they are, (4 or 5, more?) but they always seem to be the wrong one, if trying to use another's tool on your line, or vice versa! My own for the last 40 years have been what I think are called 'type G", not sure about that, but I know with all my tools etc. I'm not going to switch now. I have a fitting on one of my air tanks, so I can run a nail gun off the truck's air system, every time I've offered that, when the carpenters had an issue with their compressor, wrong fittings!