These socket wrenches are made of a special chromed alloy steel. The socket walls are made thin to make it easier to fit to bolts or nuts that are set in narrow, difficult to reach spots. The nominal sizes of the sockets are determined by the socket opening, not the bolt size. When ordering, please indicate the size of the square drive opening at the back of the socket: 6.35 mm (1/4″), 9.5 mm (3/8″), 12.7 mm (1/2″), 19.0 mm (3/4″), 25.4 mm (1″). Also specify the number of points in the socket and the size of the opening. The following illustrations show how to measure the opening of the socket sizes properly. Careful When Ordering! American and Metric wrench sizes are taken across the flats of the bolt head, British Standard size is taken from the outside diameter of threads.