From Tekkotsu Wiki

• d = height of reference frame n's origin above the x_n-1-y_n-1 plane
• θ = x_n's angular offset (about the z_n-1 axis) from the x_n-1 axis
• r = distance to travel from z_n-1 along the x_n axis to reach the origin of reference frame n
• α = z_n's angular offset (about the x_n axis) from z_n-1

When z_n is parallel to z_n-1, the value of d is arbitrary, and α is 0.

Another Illustration

This version uses a slightly different order of operations: θ, d, r (called a), and α. Switching the d and θ steps has no effect on the final result.

Step 1: Rotate around z0 by θ
Step 2: Displace along z0 by d
Step 3: Displace along x1 by a (or r)
Step 4: Rotate around x1 by α
All in one