Abstract: A substantially planar element for use in building-up a model structure by hingedly and detachably connecting a plurality of such elements in edge-to-edge relationship, whereinA. the element is of geometric shape having at least one margin,B. said margin comprises at least two projections having a recess therebetween,C. an end face of one projection is formed with a recessed hinge socket,D. an end face of the other or another projection is formed with a protruding hinge pip,E. an end face of one of the projections is an abutment face,F. the margin is adapted to mate with a complementary margin of another such element with the hinge pip of each element sprung into the hinge socket of the other element and with the abutment face abutting the complementary abutment face of the other element so as to maintain the hinge pip of each element in the hinge socket of the other element.