Relative Instantaneous Centre of Rotation:

If both of the bodies 1 and 2 are in motion, the instantaneous centre may still be resolute in the similar manner as in the previous section. This instantaneous centre is known as relative instantaneous centre or it is also known as Centro. It is indicated as P₁₂. This point P₁₂ contains zero relative velocity w. r. t. a coincident point on body 1. As a consequence, the relative motion of body 2 w. r. t.  body 1 seems to be pure rotation around P₁₂ at this instant. This is evident that P₁₂ & P₂₁ are similar. It can be noted that the instantaneous centre may lie anywhere in the plane of motion inside or outside physical boundary of bodies (links) however can be referred to be integral points of the concerned either of two bodies. If this lies outside the boundary, the boundary of the body may be imagined to be extended.