A mechanism which constrains the spatial location of a working or a focal pointof a tool (103), has a manipulator (101) and a remote centre mechanism (102) mountedthereon. The manipulator (101) provides at least one degree of freedom for positioningthe remote centre mechanism (102) which itself provides at least a further degreeof freedom for positioning a tool holder suitable for holding the tool (103) whichis to be constrained in operation with respect to a remote centre point. The orientationof the tool holder is adjustable whilst still maintaining the position of theremote centre point fixed with respect to the manipulator (101).
