| dcu | Delphi Compiled Unit |
| dcuil | Delphi .NET Compiled Unit |
| dda | GLBasic ASCII Scene Data |
| ddb | ProWorx NXT Descriptor Data with Blocking |
| ddd | Alpha Five Data Dictionary |
| ddf | Diamond Directive File |
| ddf | ProWorx NXT Descfriptor Data File |
| ddl | Data Definition Language File |
| ddm | Alpha Five Data Dictionary Memo |
| ddp | Delphi Diagram Portfolio |
| ddw | GLBasic World File |
| ddx | Alpha Five Data Dictionary Index |
| dec | VersaPro Declaration File |
| dectest | Python Decimal Arithmetic Based Test Cases Data |
| def | C++ Definition File |
| defs | BlueJ Settings File |
| dem | Turing DEM File |
| dem | Delphi Edit Mask |
| deo | Creator Simulator Compiled Module |
| dep | Visual Studio Package and Deployment Wizard Dependency File |
| depend | Code::Blocks Dependencies File |
| deployproj | RAD Studio Deployment Manager Configuration |
| des | Creator Simulator Mozart Source Code |
| developerprofile | Xcode Developer Profile |
| dex | Android Compiled Executable |
| df | OpenEdge ABL Data Definition |
| df1 | Omnis Native Data File |
| df2 | Omnis Native Data File |
| df3 | Omnis Native Data File |
| df4 | Omnis Studio Database File |
