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Differential length represents infinitesimal change in any direction of the axis in the coordinate system and is represented by . In a Cartesian coordinate system, the differential length is given by

(1.33)

Its magnitude is defined by

(1.34)

The infinitesimal change for the surface area is defined by the differential area. The unit vector of a differential area points normal to the surface. The differential areas in a Cartesian coordinate system for the surface areas shown in Figure 1.9 are defined as

(1.35)

The infinitesimal change for the volume is defined by

(1.36)

The illustration of the changes in length, area, and volume for a Cartesian coordinate system are given in Figure 1.9.