Module 3 - Phonetic Alphabet & Prowords
Numbers and Critical Digits
What you will learn
Use the IMO modified pronunciations for digits that are easily confused and keep long identifiers grouped without changing their order.
The IMO table modifies the pronunciation of several digits so they remain distinct internationally: ZEERO, WUN, TOO, TREE, FOWER, FIFE, SIX, SEVEN, AIT and NINER.
The modified forms are an aid to intelligibility, not a licence to change the number. Work from the written value, speak one digit at a time and keep the original order. A nine-digit MMSI must remain nine digits from the first group to the last.
IMO also prints TOUSAND for 1000. For 2.5, its numbers section shows both two decimal five and two point five. This module uses decimal in drills because it is distinctive, while recognising the other documented form.
Pair every number with its meaning. One-six might be a channel, but the receiver should not have to infer that. State Channel one-six, bearing one-three-seven degrees or distance two decimal four nautical miles as appropriate.
| Number | IMO pronunciation | Number | IMO pronunciation |
|---|---|---|---|
| 0 | ZEERO | 5 | FIFE |
| 1 | WUN | 6 | SIX |
| 2 | TOO | 7 | SEVEN |
| 3 | TREE | 8 | AIT |
| 4 | FOWER | 9 | NINER |
| 1000 | TOUSAND | 2.5 | TOO DECIMAL FIFE or TOO POINT FIFE |
Protect the order of an identity number
You must read the fictional MMSI 232019450 from a card during a training exercise.
1. Read from the card
Do not rely on memory when the written station identity is available.
2. Speak separate digits
TOO TREE TOO — ZEERO WUN NINER — FOWER FIFE ZEERO, using brief grouping pauses without adding or dropping digits.
3. Verify the count
Check that all nine digits were transmitted in the original order and repeat if requested.
Sense check: Grouping can help the listener write the number, but it must never alter the registered identity.
Common mistake or limitation
- Changing the pronunciation but accidentally changing the digit order as well.
- Saying a bare number without identifying whether it is a channel, bearing, time, distance or identity.
- Teaching decimal as the only documented form for 2.5 even though IMO also prints point.
Recap
- Use the published modified pronunciations for clear international speech.
- Keep identifiers in their original order and verify the digit count.
- State what the number represents and include its unit where needed.
- For 2.5, both decimal and point appear in IMO SMCP.
Optional quick check
Section 5 of 7
How is 150 normally spoken under the IMO separate-digit rule?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Phonetic Alphabet & Prowords
This public lesson is part of Revision Module 3. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.