| AutoCAD Undo Info Format | |
| Autodesk | |
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File with an .AC$ extension contains data used by different instructions for AutoCAD, stored temporarily and automatically by the software during its running.
AC$ files do not contain any information on the image, they are used by certain commands, such as HIDDEN LINES or UNDO, and they are deleted after program stops running.
Windows
| .aawdef | Ad-Aware Definitions Format |
| .abr | Adobe Photoshop Brush Format |
| .ac$ | AutoCAD Undo Info Format |
| .acc | DR DOS - ViewMax Format |
| .acl | Microsoft Office Automatic Correction List |
| .acs | Microsoft Agent Character Structured Storage Format |
| .ad | After Dark Screen Saver Format |
| .add | Microsoft Dynamics AX Developer Documentation Format |