The Aronhold Kennedy Theorem states that **if three bodies move relatively to each other, they have three instantaneous centres and lie on a straight line**.

Consider three kinematics links A, B and C having relative plane motion. The number of instantaneous centres (N) is given by

N = n(n-1)/2

n = Number of links = 3

∴ N = 3(3-1)/2 = 3