AC$ File

.ac$ is a temporary working file created automatically by AutoCAD during an active drawing session. It stores undo state information - the intermediate data AutoCAD needs to reverse editing operations when the user runs the UNDO command. AutoCAD writes this file alongside the open DWG drawing or in the system temporary directory, then deletes it automatically when the session closes normally.

Because .ac$ is a runtime scratch file, it is never intended to be opened directly. If AutoCAD terminates unexpectedly - due to a crash or a forced shutdown - the .ac$ file may persist on disk as an orphan. It can be safely deleted in that case; it holds no recoverable drawing content and is not a substitute for AutoCAD's dedicated .bak backup or .sv$ autosave files.

The .ac$ extension is also associated with AutoCAD AutoSpool, the application's legacy print-spooling component, where it temporarily buffers plot data queued for output.

Like other AutoCAD temporary files - .bak backups, .sv$ autosave files, and .dwl drawing lock files - .ac$ files appear and disappear as part of normal AutoCAD operation and should never be archived or distributed.