Abstract: A junction member (1), has an external form (2) that is polygonal in cross section and has external faces (4) of a polygon; adjacent corners (3) of the polygon defining respective external adjacent corners of each external face, each of those corners constituting a respective tunnel (5) so that each corner is a respective corner box section; and an internal form (6) that is polygonal in cross section and constitutes an internal box section, each corner box section intersecting a respective corner (7) of the internal box section. The external face has one slot (8) between respective external adjacent corners, the base of the slot being comprised by a respective face (9) of the internal form, said slot being a subtantially T-shape so that a suitable tenon may be inserted into the said slot, optionally thereby permanently or releasably to interlock with the junction member.