To enable the contour of a recess in a piece of material, which is worked into the material by means of a tool, in particular in a piece of bone, to also be determined without a direct view of the machining region with a method for determining this contour, it is proposed that the position of the piece of material in space is established by a navigation system, that the position of the tool in space is established by a navigation system, that the respective position of the tool in relation to the piece of material is determined from the position data obtained in this manner, that during the machining of the piece of material with the tool, the relative positions of the tool in relation to the piece of material are stored and that the prepared contour on the piece of material is determined from extreme values of these relative positions with respect to a fixed reference position of the piece of material. A device for performing this method is also specified.
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