Traditionally, since AutoCAD version 2000, Autodesk has changed its DWG file format after three versions. Since 2013, however, Autodesk decided to keep the same DWG format (AC1027) for 5 versions, until they finally decided to give AutoCAD 2018 a new DWG format (AC1032).Essentially, this means that if you save a drawing file in 2018, it will not be compatible with previous versions.

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Source-CADsoft Consoulting