Commonly used on performance motors, these install between the cylinder and the mating surface of the head.  They are used to adjust deck height.  Some builders prefer to use a combination of cylinder shims under the cylinder and copper gaskets at the top to eliminate or minimize stacking of cylinder shims under the base of the cylinder.  They can be reused if tearing down the engine.  Available in .040", 050", and 060" thicknesses for 90.5/92mm and 94mm cylinders.  Remember, if using thick-wall 92's, you'll need the ones that fit 94's to match the OD of the cylinder.  Sold in sets of 4.