Author: Alan, 5B4AHJ
Usual practice when operating from a vessel at sea is to append /MM to the normal callsign. A prefix indicating the country of registration of the vessel may also be added.
Some administrations have issued specific prefixes for use when operating maritime mobile.
These are elisted in the table below.
| Administration | Prefix | Source | Note |
| Canada | VE0 | From 1954-01-01 | |
| Canada | XJ0 |
From 1975-09-01. XJ0NEH & XJ0MAD known to have been QRV. | |
| Ecuador | HC9, HD9 | NE8Z | Used as early as 1967, and probably earlier. See HC9CA |
| Liberia | EL0, 5L0 | G0CUZ | Used as early as 1956. and probably earlier. See EL0B |