An apparatus and measuring instrument includes a first part and a second part slidably moveable with respect to one another. The first part includes a locking member that has a hard portion which is movable from a first position to a second position. The second part includes a compressible portion that has a Shore hardness less than the hard portion and is positioned such that when the locking member is in the first position the compressible portion is not engaged by the hard portion, and when the locking member is in the second position the compressible portion is at least partially compressed by the hard portion thereby preventing relative movement of the first and second parts.
