Fahrenheit (^oF): Defined so that the freezing and boiling points of water are 32^oF and 212^oF, respectively.

Celsius (^oC): Defined so that the freezing and boiling points of water are 0^oC and 100^oC, respectively.

Kelvin (^oK): Defined so that the absence of thermal energy corresponds to 0^oK.

• A difference of 1^o is the same in Celsius and Kelvin.
• The freezing and boiling points of water are 273^oK and 373^oK, respectively.

Only the Kelvin scale is fundamental because it is proportional to thermal energy. Physics formulae involving temperature make sense only in the Kelvin scale.