Determination of Relative Pole for Four bar Chain:

The relative pole of AB associated to O₂A is at A. The relative pole of O₄B connected to O₂A is at R₁₂. It is find out by the following process:

 (a) Join O₂ O₄ and extend it. Rotate O₂ O₄ around O₂ through the angle (θ₁₂) /2 in a direction opposite to that of O₂ A where θ₁₂ is angle of rotation of O₂A in clockwise sense.

(b) Rotate O₂ O₄ around O₄ through an angle   (φ ₁₂)/2 in a direction opposite to that of O₂ B here φ₁₂ is angle of rotation of O₂ B in clockwise direction. Extend it until meet the primary position.

 (c) The point of intersection of these two positions is the wanted relative pole R₁₂.

It is illustrated in Figure. It may be proved that the angle subtended through O₂ O₄ at R₁₂ is equivalent to the angle subtended through the coupler at R12.

As angle subtended by O₄  O₄′ and B₁ B₂ are similar.

i.e.

< O₄ R₁₂  O₄′      = B₁  R₁₂  B₂

or,

< O₄ R₁₂  O₂  = B₁  R₁₂  A