Considering first link P 1 -A: all points on this link, including point A, rotate around point P 1. Since P 1 is the only point not moving in the given plane it may be called the instant center of rotation for this link. Point A, at distance P 1 -A from P 1, moves in a circular motion in a direction perpendicular to the link P 1 -A, as indicated by vector V A.