Reference points (Latin referre = to refer to) represent a special value to which calculations and measurements refer. In order to determine the relationship of two quantities to each other, two reference points are necessary for a linear relationship, and more for a nonlinear relationship, depending on the complexity. In order to be able to indicate a geometrical place vector correctly, a zero direction must be defined to a unique reference point usually also to the orientation. Zero points represent special reference points with a value of 0 as starting point for measured, calculated or to be represented values. Reference points can be, as for example in the case of the spherical coordinates, also zero points of coordinate systems. Reference points and reference systems are essential references for correct measurements or machining of objects. According to the applicable standards, a datum is defined as "one or more situation features of one or more features associated with one or more real integral features selected to define the location and orientation, or both, of a tolerance zone or ideal feature representing, for example, a virtual condition".