Gauge blocks and sets are commonly stacked together to make a desired length. Rectangular gauge blocks, also known as Jo blocks, are made of metal or ceramic. They have two opposing faces that are ground flat at an exact distance apart. Square gauge blocks, also known as Hoke blocks, are metal or ceramic with two opposing faces at an exact distance apart. Angle gauge blocks are built with a right angle in one corner and are used for taking measurements of angles. Cylindrical gauge blocks are round and flat and have a hole in their center and most commonly used as spacers or as setting standards. Gauge balls are perfectly round and used to check the depth and outer diameter of tapered shafts.