What is the Unified Screw Thread System and how does it relate to TPI? The Unified Thread System is a standardized system adopted by the United States, Canada, and Great Britain that unifies the thread specification of different screw sizes. The TPI is included in these specifications along with the coarseness or fineness of the thread. In fact, the level of coarseness and fineness of the individual threads directly impact the TPI. A course thread will result in a lower TPI while a fine thread will result in a higher TPI. The individual specifications included in the United Screw Thread System include: UNC, which stands for a course thread, UNF, which stands for a fine thread, UNEF, which stands for an extra fine thread, and UNS, which stands for a unified special thread.