Take two pieces of wood of the same dimensions. On each, cut two slots as far apart as the pieces of wood are wide through to half the thickness, then chisel out the wood between the slots. Smear the "holes" with glue, turn one piece of wood up the other way and turn it 90°, then join the two pieces together so that the "holes" mate. You have made an < i>assemblage à mi-bois. This forms a cross, but the same can be done anywhere, at corners, with square cuts or mitres, to make tees, with the "leg" of the tee extending to the far side or only partially through the other part, etc.

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Note added at 2 hrs (2009-04-05 14:55:09 GMT)
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http://woodworking.about.com/od/joinery/p/halfLapJoints.htm
http://www.diydata.com/techniques/timber_joints/frame_joints...
[both illustrated]