Abstract: The joint between two nearly rigid, mating walls has a tongue on one wall's surface and a groove on the other wall's surface which have special cross section shapes cooperating to resist penetration of the joint by fluids such as water. At least one of the walls is formed of a plastic material which deforms under compressive loading. The shape of the tongue includes sides thereof which intersect the end to form corners defining the longitudinal edges of the tongue. The groove has walls which converge toward each other at the bottom of the groove from a mouth wider than the width of the end of the tongue. The shape of the groove forms a line of contact with each of the tongue's corners when the tongue is mated with the groove. During assembly, the corners of the tongue are forced into the walls of the groove by some type of clamping mechanism so as to at least slightly deform at least one of the tongue and groove.