a non - releasable mechanical join useful for permanent joins of plastics material cartons is formed by a row of a plurality of engagement tabs interlocked in a complementary row of slots . the engagement tabs have heads able to passthrough the slots in one direction but unable to return in the other direction . the tab heads may be formed as arrowheads , the barbs of which act against edges of the slots to disallow withdrawal . suitably , the leading edges of the slots ar convexly arcuate towards the approaching arrowheads . this may help to provide secure engagement . the greater the number oftabs and slots , the greater may be be the permanence of the join .