Auxiliary device for placing elements in the form of a plate comprising a support element and a pressing element wherein: the support element comprises a substantially flat base (1.1) and a grip rod (1.2) extended from said base (1.1) perpendicularly thereto, where the base (1.1) is adapted to support at least two adjacent elements in the form of a plate wherein said elements in the form of a plate remaining separated at a distance corresponding to the separation between opposed larger faces (1.21) of the grip rod (1.2); the pressing element is adapted to be coupled to the grip rod (1.2) and to fasten the elements in the form of a plate between the support element and said pressing element to achieve coplanarity between said elements in the form of a plate, wherein the grip rod (1.2) is connected with the base (1.1) by means of a weakened portion (1.3), adapted to be broken when an attempt is made to pull the grip rod (1.2) and the base (1.1) offers resistance; wherein the grip rod (1.2) further comprises smaller lateral faces (1.22) which in turn comprise two rows of external nut threads (1.221) which result in segments of a nut thread developed helically about an axis of rotation perpendicular to the base (1.1); and wherein the pressing element has the form of: a threaded ring (3.1) comprising an internal nut (3.11) adapted to screw tightly into the rows of external nut thread (1.221) of the grip rod (1.2); or a wedge (3.2) arranged to be inserted parallel to the base (1.1) into a longitudinal opening (1.23) that traverses the larger faces (1.21) of the grip rod (1.2); characterised in that the grip rod (1.2) has the form of a rectangular bar and the longitudinal opening (1.23) is rectangular, and its dimensions correspond to a maximum rectangular cross-section of the wedge (3.2); and in that an upper surface (3.21) of the wedge (3.2) comprises a plurality of parallel coupling grooves (3.211) adapted to be set into an upper surface (1.231) of the longitudinal opening (1.23) in the form of a "V"; and in that once the adjacent edges of the elements in the form of a plate have been arranged on the base (1.1), the wedge (3.2) is arranged to slide over said elements in the form of a plate, through the longitudinal opening (1.23), until the upper surface (3.21) of said wedge (3.2) makes contact with the grip rod (1.2). the support element comprises a substantially flat base (1.1) and a grip rod (1.2) extended from said base (1.1) perpendicularly thereto, where the base (1.1) is adapted to support at least two adjacent elements in the form of a plate wherein said elements in the form of a plate remaining separated at a distance corresponding to the separation between opposed larger faces (1.21) of the grip rod (1.2); the pressing element is adapted to be coupled to the grip rod (1.2) and to fasten the elements in the form of a plate between the support element and said pressing element to achieve coplanarity between said elements in the form of a plate, a threaded ring (3.1) comprising an internal nut (3.11) adapted to screw tightly into the rows of external nut thread (1.221) of the grip rod (1.2); or a wedge (3.2) arranged to be inserted parallel to the base (1.1) into a longitudinal opening (1.23) that traverses the larger faces (1.21) of the grip rod (1.2);