There are a few rules to consider in choosing the coordinate system:
 the axis is in the direction of the joint axis
 the axis is parallel to the common normal: If there is no unique common normal (parallel axes), then (below) is a free parameter.
 the axis follows from the  and axis by choosing it to be a righthanded coordinate system.
Once the coordinate frames are determined, interlink transformations are uniquely described by the following four parameters:
 : angle about previous , from old to new
 : offset along previous to the common normal
 : length of the common normal (aka , but if using this notation, do not confuse with ). Assuming a revolute joint, this is the radius about previous .
 : angle about common normal, from old axis to new axis
“Standard” DH Parameters
Following the DH standard you must provide 4 numbers that define the orientation of the link with respect to the link. “Standard” DH convention assumes that the coordinate frame is at the . (joint 1 axes 0, joint 2 axes 1 …)
 (Link parameter)()The first number represents the angle (in radians) between and about .
 (Link parameter)()The second number represents the length (in meters) along of the common perpendicular between and .
 (Joint parameter)()The third number represents the angle (in radians) between and about .
 (Joint parameter)()The fourth number represents the distance (in meters) along axis between the origin of the coordinate frame and the point where the common perpendicular intersects axis
“Modified” DH Parameters (also called Craig’s convention)
Following the modified DH standard, you must provide 4 numbers that define the orientation of the link with respect to the link. Unlike the “standard” DH convention, the “modified” DH convention assumes that the coordinate frame is at the . (joint 1 axes 1,joint 2 axes 2 …)
 (Link parameter)()The first number represents the angle (in radians) between and about .
 (Link parameter)()The second number represents the length (in meters) along of the common perpendicular between and .
 (Joint parameter)()The third number represents the angle (in radians) between and about .

(Joint parameter)()The fourth number represents the distance (in meters) between and about .
Many people in robotics are actually unaware that there are two different conventions in use. An advantage of Craig’s convention is the proximal placement of the origin for a link. Also the rotation is about and the joint number is the same as the coordinate number, which seem more natural. Torque exerted about joint is also at the same place as at link ’s coordinate system, to which inertial parameters such as center of mass are likely to be referenced. A disadvantage is that the transform mixes and parameters. Both Craig’s convention and the standard DH convention are equally valid. The choice of one over the other is merely a matter of taste or habit.