| Code | Name | Region |
|---|---|---|
| US5EI | Clemons | IA |
| USCEU | Clemson | SC |
| USIUH | Clendenin | WV |
| USXER | Cleona | PA |
| USCFW | Clermont | FL |
| USLMN | Clermont | GA |
| USERN | Clermont | IA |
| USCM7 | Clermont | IN |
| USCLO | Clermont | KY |
| USJC5 | Clermont | NJ |
| USCXN | Clermont | NY |
| USCE5 | Cleveland | AL |
| USCQV | Cleveland | GA |
| USCL5 | Cleveland | IL |
| USCWX | Cleveland | MO |
| USCVD | Cleveland | MS |
| USCVT | Cleveland | NC |
| US3CV | Cleveland | NY |
| USCLE | Cleveland | OH |
| US7CS | Clear Lake Shores | TX |
| US2CP | Clear Spring | MD |
| USU7Y | Clearfield | IA |
| USZCL | Clearfield | PA |
| USCQU | Clearfield | UT |
| USCIZ | Clearing | IL |
| USEAK | Clearlake | CA |
| USCLC | Clearlake | TX |
| USRXO | Clearlake Oaks | CA |
| US2PA | Clearville | PA |
| USCLW | Clearwater | FL |
| USAWT | Clearwater | ID |
| USJWA | Clearwater | KS |
| USEWG | Clearwater | MN |
| USCWR | Clearwater | NE |
| USVWA | Clearwater Beach | FL |
| USCBN | Cleburne | TX |
| USC2E | Clementon | NJ |
| USCLZ | Clements | CA |
| USCQY | Clayton | DE |
| USCN3 | Clayton | GA |
| USYTQ | Clayton | ID |
| USCKY | Clayton | IL |
| USCKZ | Clayton | IN |
| USYTN | Clayton | MI |
| USCJD | Clayton | MO |
| USCME | Clayton | NC |
| USCYN | Clayton | NJ |
| USCAO | Clayton | NM |
| USCLY | Clayton | NY |
| USCYT | Clayton | OH |