Extremely interesting explanation Sean, thanks a lot. These ones look basically the most interesting parts to me (architecture & pure 2D work): Part 202 - Associative draughting. 2D/3D drawing with association, but no product structure. http://www.steptools.com/support/stdev_docs/express/ap202/html/index.html Part 225 - Building elements using explicit shape representation http://www.steptools.com/support/stdev_docs/express/ap225/html/index.html The steptools.com site has quite a lot of documentation about step: http://www.steptools.com/library/standard/ And the XML-way a step file is written looks pretty easy to read: http://www.steptools.com/support/stdev_docs/express/ap202/demos/geometry_out_p28.html The 202 is easier to get a good idea of. Actually everything you find in a "traditional" (with a name that begins with auto and that ends with cad) 2D CAD app is there... And it's pretty complete. Actually I even believe much of the terminology comes directly from autocad. Couple of interesting screenshots: http://www.microcad.co.jp/STEP-E/products/NSviewer/detail/ap202.html And portions of AP202 step data here: http://www.ocf.or.jp/top/event/pdf/1999step3.pdf I think with the contents of that AP202 part, one could describe a 2D cad drawing with about the same level of complexity as autocad. Couple of things I didn't find are layers (but of course there must exist some form of grouping objects, probably much better than layers) and more printing-oriented stuff like line thickness, etc. But I suppose that's not the focus of the format. Now, in freecad we have pretty good step import/export (it's built-in with opencascade). I know it behaves fine with brep 3D things ( which is the main focus of opencascade) but I'd be curious to do some tests with step files containing 2D data formatted as in the AP202... The difficulty will probably be to find such files... If anyone happens to encounter one, I'd be interested to have a look at it. Cheers Yorik