It seems you might be asking about the markings "F" and "C" on a thermometer. These stand for Fahrenheit and Celsius, which are two different scales used to measure temperature.

* Fahrenheit (°F) is the scale primarily used in the United States.

* Celsius (°C) is the scale used in most other parts of the world and in scientific applications.

The markings on a thermometer indicate which scale is being used. So, when you see "F" on a thermometer, it means the temperature is being measured in Fahrenheit. When you see "C", it means the temperature is being measured in Celsius.