Boards, whether or of wood or man made materials, have downwardly converging sides in the manner of an inverted trapezoid to create a downwardly expanding space between such boards when used for decking, porches, balconies, walkways and the like. Specifically, the difference between the refractive index of the filler and the refractive index of the adhesive resin is at least 1. The cap is removably coupled to the receptacle and includes a recess that receives a portion of the slug so that the cap and slug are able to rotate relative to each other.