Every object in space will have 6 DOF (Degrees Of Freedom).They are 3 translational and 3 rotational degrees of freedom.3-2-1 Principle describes ways of constraining the 6 degrees of freedom of an object in space.By bringing the object in contact with Primary datum at minimum 3 points of contact,3 degrees of freedom are constrained.As the object comes in contact with secondary datum at minimum 2 points of contact,2 degrees of freedom are constrained.Contact of the object with tertiary datum at one point can constrain 6th degree of freedom i.e.,translation along y axis.