A mechanism which constrains the spatial location of a working or a focal point of a tool (103), has a manipulator (101) and a remote center mechanism (102) mounted thereon. The manipulator (101) provides at least one degree of freedom for positioning the remote center mechanism (102) which itself provides at least a further degree of freedom for positioning a tool holder suitable for holding the tool (103) which is to be constrained in operation with respect to a remote center point. The orientation of the tool holder is adjustable while still maintaining the position of the remote center point fixed with respect to the manipulator (101).
