Washered butt hinges have metal washers between the joints on the knuckle which helps the hinge to rotate without wearing the actual hinge. Light periodical oiling is recommended to avoid wear. 75mm hinges are ideal for standard internal doors up to 35mm thick, whereas the 100mm size is suited to 40-54mm thick doors such as fire doors and external timber doors. Check the hinge page for suitability for your doors. Ideally, 3 hinges fitted to a door will help avoid a warped door, especially airing cupboards and bathroom doors where heat and humidity can alter the shape of wood.