1925 Country Prefixes
| United States of America | N | |
| British Empire | G | |
| France | F | |
| Italy | I | |
| Japan | J | |
| Hedjaz | A-H | |
| Nicaragua | A-N | |
| Albania | B-A | |
| Bulgaria | B-B | |
| Latvia | B-L | |
| Bolivia | C-B | |
| Cuba | C-C | |
| Estonia | C-E | |
| Switzerland | C-H | |
| Portugal | C-P | |
| Roumania | C-R | |
| Uruguay | C-U | |
| Estonia | E-A | |
| Ecuador | E-E | |
| Haiti | H-H | |
| Hungary | H-M | |
| Netherlands | H-N | |
| Siam | H-S | |
| Costa Rica | K-C | |
| Finland | K-S | |
| Czechoslovakia | L-B | |
| Guatemala | L-G | |
| Liberia | L-L | |
| Luxembourg | L-U | |
| Spain | M-A to M-N | |
| Monaco | M-O | |
| Belgium | O-B | |
| Peru | O-P | |
| Brazil | P-B | |
| Poland | P-P | |
| Sweden | S-A | |
| Greece | S-G | |
| Panama | S-P | |
| Denmark | T-D | |
| China | X-C | |
| Honduras | X-H | |
| Serbia-Croatia-Slavonia | X-S | |
| Lithuania | Z-L |
Using a group of 4 letters out of the 26 of the alphabet
Each group containing at least one vowel eg: ADCJ PURN
The letter 'Y' is considered to be a vowel
Annex to Convention of Regulation de la Nav.Aerienne Paris 13.10.19
with the decision of CINA dated 13.7.22, 25.10.22, 28.2.23, 26.6.23 and 3.3.24