Final answer:
The letters "TR" on a receptacle represent a tamper-resistant outlet, designed for safety by blocking foreign objects from being inserted. It works with a standard three-hole outlet to ensure proper function of the three-wire system.
Step-by-step explanation:
The letters "TR" on a receptacle indicate that it is a tamper-resistant outlet. Tamper-resistant outlets are designed to prevent foreign objects from being inserted into the receptacle, providing an important safety feature, particularly in households with young children. Such outlets contain internal shutters that block the slots and only open when equal pressure is applied simultaneously to both sides, as is the case with a proper three-prong plug. The standard three-hole outlet ensures that the plug can only be inserted one way to assure the proper function of the three-wire system, which includes live/hot, neutral, and ground wires, each with its own color codes that vary internationally.