For a description of the Autodesk DXF format open AutoCAD Help, choose the "Customization Guide", Appendix C - DXF Group Codes; resp. the "DXF Reference" (separate help item). Specification of the DXF format of AutoCAD Rel.13 up to 2006 can be also found online at www.autodesk.com/dxf. Thanks to CADForum.cz for the info!

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