Abstract: In dental surgery when a tooth is to be crowned, the tooth has to be prepared by cutting a taper. It has been found that for the successful mounting of a crown the taper must be much more exactly prepared than is possible using currently available equipment. The present invention allows the initial position of the axis of a cutter to be related by means of electronic "spirit levels" to a selected axis. Deviation from this axis by more than a predetermined amount is indicated by light sources which can be seen regardless of the position of a handpiece holding the cutter. By means of a code these light sources indicate how the dentist must move the cutter to return to the correct axis.