Abstract: Instrument for obtaining a reference line to realize, in correspondence of this line, working points, said working points being preferably situated in correspondence of a working surface. The instrument includes: a laser ray emitting device (1), a supporting body (13), connecting means which attach it to a fixed reference surface. These connecting means include a hinge element (2) which enables to orientate the supporting body (13) and direct the laser ray and to keep it fixed in the desired direction in such a way that with the line defined by the laser ray it is possible to find the working points.