Tolerance is defined as the total permissible variation of a size. It is the difference between maximum limit and minimum limit of size.

Algebraic difference between a size and a corresponding basic size is called deviation. Limit deviations are upper and lower deviations. Algebraic difference of actual size with basic size is called actual deviation.
Upper Deviation: It is the algebraic difference between the maximum size and the basic size.
The upper deviation of a hole is represented by a symbol ES (Ecart Superior) and of a shaft, it is represented by es.
Lower Deviation: It is the algebraic difference between the minimum size and the basic size. The lower deviation of a hole is represented by a symbol EI (Ecart Inferior) and of a shaft, it is represented by ei.