| cir | Altair FEKO SPICE Circuit Data |
| cir | Cadence PSpice Circuit Data |
| cir | Ngspice Circuit Data |
| cir | Racelogic PerformanceBox Circuit Overlay |
| circ | Logisim Circuit Data |
| cis2 | CIMSteel Integration Standard 2 Document |
| citygml | CityGML Data |
| ciw | EndNote Web of Science Export |
| cix | IBM Rational XDE Collaboration Instance File |
| cjb | Autodesk Cleaner XL Job |
| cjd | Casio EX-word Dictionary Data |
| cjk | Latex CJK Package Data |
| cjstyles | Skin Framework Style Data |
| ck9 | Cook'n Cookbook Backup |
| cka | Comfort Keys Database |
| cki | Ansys Chemkin Kinetic Data |
| cki | Off by One Web Browser Cookie Data |
| ckl | DISA STIG Checklist |
| ckn | Disk Doctors CK Support Info File |
| ckp | Ingres Database Checkpoint Data |
| ckp | PSI-BLAST Checkpoint Profile Data |
| cks | PSI-BLAST Checkpoint Sequence Data |
| ckt | Ansys Chemkin Thermodynamic Data |
| ckv | CheckVision Database |
| cky | DSOrganize Data |
| ckz | QuickSFV Checksum Database |
| cl | Carlson Survey Centerline File |
| cl | IconUtils Cursor Library |
| cl2 | Hy-Tek Meet Manager Results Data |
| cl7 | Free Clipboard Viewer Saved Data |