To manufacture accurate lengths, rectangular and square gauge blocks are used individually or combined together. Gauge blocks, also known as slip gauges, are metal or ceramic rectangular (also known as Jo blocks) or square (also known as Hoke blocks) blocks with two opposing faces ground flat and parallel at a specific distance apart. Because square blocks feature a hole, they can be used with a rod to achieve vertical positioning. Gauge blocks are frequently sold in sets with a variety of lengths. Machine shops, as well as quality assurance and calibration environments, frequently use gauge blocks.