Referring to your question on the nominal option and bxd at http://www.tfguild.org/ubbcgibin/ultimatebb.cgi?ubb=get_topic;f=23;t=000008

Nominal gives you a chance to enter a size reduction in the breadth and depth of a beam. For example when using timbers planed under full size. There are also bnom and dnom options that you can set for glulams since the undersizing is not uniform. So, if you use 0.5 for your nominal variation then when you enter the size of a timber it will draw it at actual size.

This allows us to refer to timbers in their nominal size specification and draw with accuracy. You have to enter timber sizes in nominal sizes, because labelling and layering is confused by punctuation and slashes. Therefore, if you are drawing a 6x12 beam you must enter it as bxd or 6x12 where the x is the delimiter for the software to extract the b (width) and the d (depth). If nominal has been set to 1/2 then the resulting timber would be 5.5"x11.5". Keep in mind that this value is referenced when you extrude as well. So, not only should you be sure that you are in the proper BPW mode when drawing and extruding, but you must be sure your nominal settings are correct.