of for example plastics material especially if the element is a visible part in a machine or took for example a rail for length measuring instruments in order to protect the plastic block 9 in which the bodies 2 the prestressing steel rods 3 and if applicable the adjustment devices 5 and 8 are embedded against damage and if required to provide supplementary mechanical rigidity for the bar type constructional element the block 9 can be enclosed with a cladding which can be in the form of an one piece metallic profile section or as shown in the drawing in the form of for example two metallic angle plates 4 in the embodiment of a constructional element in accordance with the invention as described above in this case a rail 1 for measurement instruments the element involved is a prestressed element which essentially consists of a plurality of linearly configured bodies 2 made of ceramics or a ceramic like material of high compression strength and low relative density with prestressing devices of high tensile strength in this case in the form of prestressing steel rods 3 this combination in the invention of ceramic bodies with prestressing devices and possibly a mechanically solid cover enables the shape of the ceramic and constructional element profile to be optimised taking due account of the forces acting on it and the total weight or mass of the constructional element and possibly the feasibility of industrial scale production the adjustment devices for setting the initial stress in this case in the form of the screw 5 and the cover plate 8.