A jig is disclosed having a beam and a pair of L-shaped guide blocks mounted thereon, one block being rigidly secured thereto while the other block can be moved along the beam and then tightened thereon at any adjusted spacing from the fixed block. Each guide block has substantially parallel opposite side surfaces, and top and edge reference surfaces extended therebetween and disposed normal to one another to meet across an interior corner. In use, the top and edge reference surfaces of the jig are adapted to be butted flush against the top and edge surfaces of each respective board. The fixed guide block has a single drill guide opening and the slidable guide block has two parallel drill guide openings, each guide opening being formed through and normal to the guide block edge surface in a direction parallel to the side surfaces and to the top reference surface and being spaced from the top reference surface a single identical distance "t". The two drill guide openings in the sliding guide block are spaced apart a distance "s" and each of these openings and the drill guide opening in the fixed guide block is spaced from the respective adjacent side surface of the block a similar distance "d".
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