Rigid tubing fittings work exclusively with rigid tubing, though it can be of any form. Acrylic, PETG, plated brass, copper, and glass tubing will all work with rigid tubing fittings provided the fitting size matches the outside diameter (OD) of the tube. Rigid tubing fittings hold the tubing inside by compressing one or more o-rings in between the tube and the fitting securing the tube in place. Some have external o-rings that are compressed when the cap is tightened, others simply slide over the end of the tube and rely on internal o-rings to maintain a seal. Unlike flexible tube fittings, the inside of the tube isn’t in contact with anything, so the inside diameter (ID) of the tube doesn’t matter. TITAN RIG offers the most advanced rigid fittings in the industry from many different brands in a range of colors that’s sure to have what you need for your water cooled PC.