a runner 4 a as shown in fig1 13 and 14 with engagement pieces 24 longitudinal runners 4 may serve as cross runners an upward flange 27 on the engagement piece 26 and central web embracing notch 28 serve to stabilize such pieces 26 on the ends of the runner 4 a when either form of cross runner 23 4 a is fitted to a longitudinal runner 4 the lower faces 25 of both runners will be approximately coplanar 0063 fig1 and 16 show the abutting connection of the ends of runners 4 using a pair of dowels 28 the dowel diameter is preferably selected to fill together with the peripheral edge 13 and web 14 of a runner 4 the interior space 8 within a clamp 3 this ensures the true alignment of adjacent runners 4 in fig1 an enlarged detail of an arrow headed peripheral edge 13 grasped by lips 7 of a clamp 3 is depicted the runner 4 is in a bi stable orientation when canted sideways because the lips 7 must separate at least slightly if the runner 4 is rotated in either lateral direction a variety of cross sectional shapes can be provided for the peripheral edge 13 to meet this criterion in fig1 the side portions 12 a are conveniently angled to lie against the underside face of the arrowhead in near parallel alignment for maximum stability in fig1 a joining plate 30 is shown.