 Temperature is measured with a thermometer.Thermometers are calibrated in various temperature scales that historically have used various reference points and thermometric substances for definition.The most common scales are the Celsius scale (formerly called "centigrade", denoted °C), the Fahrenheit scale (denoted °F), and the Kelvin scale (denoted K), the last of which is predominantly used for scientific purposes by conventions of the International System of Units (SI).