The Futures quad rear shims are designed to let you use 1/2″ rear fins in 3/4″ boxes. Simply place the shim in the 3/4″ rear boxes to fill the extra space.

During surfboard manufacture with Future fins, two fin box depths, 1/2 and 3/4 inch, are used. 1/2 inch boxes are primarily designed to be installed at the rear of the board where the board is thinner. Forward fins use the 3/4 box depth. 

These shims are designed to go into your 3/4 inch box, therefore allowing you to use the extensive range of quad rears as side bites.  Increasingly your fin choice immensely for side bites.