Abstract: An element for use with others of a like kind in the prefabrication of structures such as sliding clasp fasteners, comprising a hexahedral body of rectangular cross-section, the two faces of one of the two pairs of parallel faces defining said cross-section being of unequal length with that of relatively greater length having therein a longitudinal parallel-sided groove penetrating the remaining two faces of the body, and a parallel-sided tenon similar in width to said groove projecting from at least one of said remaining two faces.