The temperature of a body is measured indirectly by making use of the fact that certain physical property of a substance changes linearly with the temperature

If the value of the thermometric property X differ linearly with temperature T, then selecting the simplest possible relationship

T(x) = aX + b

Where the constants b and a must be measured. This linear scale seems that each interval of temperature corresponds to the similar change  in the value of the thermometric property. To measure a temperature on this scale two calibration points are select whose temperatures are described as T1 and T2 arbitrarily. The relative thermometric property X1 and X2 are measured..