The test tube rack is used to place test tubes, and sometimes it is also used to observe some experimental phenomena. It is the most basic experimental instrument in a chemical laboratory. This 50 Wells Test Tube Rack will hold your 12-13 or 15-17mm diameter test tube securely in place. It is great for holding tubes in rack securely in place when in a water bath. Each position is alpha numerically identified. Units can be anchored laterally to one another, thanks to two screws supplied with each rack.