Two-letter groups, numbers and substitutes
1The lesson in plain words
Two-letter groups carry most Code messages. Substitutes let one set of flags repeat a flag in the same group, by a strict counting rule.
Groups to know
NC I am in distress and require immediate assistance; it is also a COLREG Annex IV distress signal. CB I require immediate assistance. AC I am abandoning my vessel. CS What is the name or identity signal of your vessel? ZL Your signal has been received but not understood. ZQ Your signal appears incorrectly coded; check and repeat the whole. UW I wish you a pleasant voyage. MAYDAY is a radio word, not a flag group.
Yes, no and questions
Send C after a group for yes or an affirmative, and N (‘NO’ by voice) to make it negative. Send RQ after a group to turn it into a question. ‘CY’ means ‘Boat(s) is (are) coming to you’; ‘CY N’ means they are not. These procedure signals cannot be used with single-letter signals.
Numbers and decimal points
Use numeral pennants for numbers. Insert the answering pennant for a decimal point. Do not count it when choosing a substitute.
The substitute rule
A ship carries one set of flags, so a flag cannot appear twice. Substitutes solve this. The first substitute repeats the uppermost flag of the class just above it, letters or numerals. The second repeats the second flag of that class, counting from the top, and the third repeats the third. No substitute may be used twice in one group. Ignore the answering pennant when it is a decimal point.
Worked examples
‘VV’: V, then first substitute. ‘1100’: 1, first substitute, 0, third substitute; the fourth flag repeats the third numeral, 0. ‘L2330’: L, 2, 3, second substitute, 0; the substitute follows numerals, so it repeats the second numeral, 3. ‘D2210’, the date 22 October: D, 2, first substitute, 1, 0.
2Step by step
Working out a hoist with substitutes
- 1
Write the group top to bottom and split it into letters and numerals.
A substitute repeats a flag of the class directly above it.
- 2
Mark every flag that repeats an earlier flag of the same class.
Only those positions need a substitute.
- 3
Count that earlier flag’s position within its class, from the top.
First, second or third gives the substitute to use.
- 4
Check no substitute appears twice in the group.
The Code forbids using one substitute twice in a group.
- 5
Read the finished hoist back from the top before hoisting.
One wrong substitute changes the whole message.
3Key data
- NC
- I am in distress and require immediate assistance
- CB
- I require immediate assistance
- AC
- I am abandoning my vessel
- ZL
- Your signal has been received but not understood
- Decimal point (flags)
- Answering pennant inserted between numerals
- Substitute n
- Repeats the n-th flag, from the top, of the class above it
4Key points
- 1
NC is distress; CB is immediate assistance; AC is abandoning the vessel.
- 2
C after a group affirms, N or ‘NO’ negates, RQ makes a question; not with single letters.
- 3
The first, second or third substitute repeats the first, second or third flag of the class above it.
- 4
One substitute may appear only once per group.
5Frequent mistakes
Wrong: The first substitute repeats the flag directly above it.
Right: It repeats the uppermost flag of the class directly above it. In ‘L2330’ the repeated 3 needs the second substitute.
Wrong: Hoist ‘MAYDAY’ in flags to signal distress.
Right: MAYDAY is a radiotelephone word. The Code distress group is NC.
Wrong: Add RQ to a single letter, such as ‘G RQ’, to ask if a pilot is needed.
Right: C, N, NO and RQ cannot be used with single-letter signals. Find the two-letter group for the question.
6Practice case & exam keys
Your master asks you to send MD 100: ‘My course is 100°’. With one flag set, a colleague lays out M, D, 1, 0, first substitute. Would you hoist it? Explain your check and any correction.
Show the answer
Do not hoist it. The first substitute repeats the first numeral, 1, so this would read MD 101. Replace it with the second substitute, which repeats 0, the second numeral. Read back M, D, 1, 0, second substitute as MD 100 before hoisting.
Exam keys
- NC distress; CB immediate assistance; AC abandoning vessel; ZL received but not understood.
- C yes, N/NO no, RQ question after a group; not used with single letters.
- Substitutes repeat the 1st, 2nd or 3rd flag of the class above; each once per group.
- Answering pennant as decimal point; disregard it when counting for substitutes.
Finish this unit
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