The invention pertains to a device for determining the relative position in the X-Y plane between two essentially flat elements that are spaced apart in the Z-direction and are essentially arranged on top of one another, wherein said device features at least one optical sensor that is arranged between the elements and is able to sense at least two points of the mutually facing surfaces of the elements, as well as an evaluation unit, in which the images of the points can be evaluated with respect to their relative position in the X-Y plane. In this case, the optical sensor features at least one line sensor that can be displaced relative to the two elements in such a way that the mutually facing surfaces of the elements can be optically scanned at least in sections in the manner of a scanner.