The invention relates to an instrument for use in computer guided surgery. The instrument includes a shaft and a reference element adapter, wherein the reference element adapter is directly couplable to the shaft and rotatable about the shaft. A selectively operable mechanical retainer provides retention of the reference element adapter to an instrument shaft adapting interface of the instrument shaft. The instrument also includes a reference element orientating mechanism arranged at an interface between the instrument shaft adapting interface and the reference element adaptor. The reference element orienting mechanism is capable of fixing at least one angular position of the reference element. The instrument also includes a grip piece interface on the instrument shaft, and a grip piece is selectively couplable to the grip piece interface.
