When two pieces are not in contact, it is assumed that any displacement of one relative to the other can be defined as a combination of 6 independent elementary motions along the axes of an orthonormal coordinate system :

• - 3 translations parallel to the reference axes, Tx, Ty, Tz,
• - 3 rotations around axes parallel to the reference axes, Rx, Ry, Rz.

It is said that there are 6 kinematic degrees of freedom (DOF) between the two pieces.

As soon as 2 pieces are in contact, a mechanical connection between the two parts modifies or forbids one or more of the 6 possible elementary movements.

Whatever the constructive solution adopted by the designer, a link (theoretical model associated with this constructive solution) is characterized from the kinematic point of view by :

- the number and nature of his degrees of freedom,

- its name, its orientation in space and the standardized symbols that represent it.