What is the .bak file?

Question
What is the .bak file that’s saved in the same folder as my .dwg file?

Answer
When an AutoCAD drawing .dwg is created or saved, typically a drawing .bak backup file is created. It’s saved in the same location as the .dwg file with the same name by default. The backup file is the last version of the current saved drawing.

If you need to recover data the .bak files can be renamed; example Grd_fl.bak to Grd_fl.dwg the open the file.

NOTE: Backup files are created only if the system variable ISAVEBACK is set to 1.