How does the zeroth law of thermodynamics make temperature measurement possible?

The zeroth law of thermodynamics is fundamental to temperature measurement because it establishes the logical basis for using thermometers to determine if objects are at the same temperature.21

The law states that if two bodies are each in thermal equilibrium with a third body, they must also be in equilibrium with each other. In practice, this allows an object—such as a thermometer—to act as that "third body". When a thermometer is brought into thermal contact with an object, it reaches thermal equilibrium with it, meaning both are at the same temperature. Because the thermometer is calibrated to reflect its own physical changes (such as variations in volume or electrical resistance) against a defined temperature scale, it provides an objective, reliable way to measure the temperature of the object it contacts. Without this principle, it would not be meaningful to use a separate device to assign a temperature value to another system.21