DWG file extension is used by DraWinG files. These files are generated and viewed by a large number of Computer-Aided Design (CAD) applications. These files contain information about about 2D and 3D objects or blueprints in other words. DWG format was originally developed in the 1970s and later licensed by Autodesk. There has been 18 major revisions of the standard so far.
You can open DWG files natively by such applications as AutoCAD, PowerCAD, Intellicad and others. Because of licensing restrictions there are no open-source software that works with it. However there are several free programs that can open DWG files and allow either viewing or conversion of DWG files. These include: AutoDWG DWG to image/PDF converter, progeCAD viewer, Volo View Express, Bentley View and others.
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