International Code of Signals — Chapters XI–XII: Single-letter Signals

1. Overview

Source: INTERNATIONAL MARITIME ORGANIZATION. International Code of Signals. London: IMO, 2005. Chapter XI — Single-letter signals (the A–Z table; Note 1, sound signalling under COLREG-72 Rules 34 and 35; Note 2, K and S as landing signals; the fishing-vessel meanings of G, P and Z) and Chapter XII — Single-letter signals with complements (azimuth/bearing, course, date, longitude, communicate, latitude, distance, speed in knots and in km/h, local time, call to shore stations, UTC).

Syllabus: Anexo 2-B, Área VI (Comunicações), item 3 — VI.03 (IMO, International Code of Signals, 2005), Capítulos XI e XII. Anexo 2-A, item VI.3.7 — Sinais de letra única.

Chapters XI and XII hold the shortest and most demanded part of the Code: twenty-five signals carried by one letter, and twelve more carried by a letter followed by numerals. Both chapters open with the same licence — they "may be made by any method of signalling" — which is what makes them the working vocabulary of a ship that has no time to spell out a message.

The economy has a price. A single-letter signal says a great deal in one character, so the difference between O (man overboard) and U (you are running into danger) is the whole message. The Code therefore fences the set with two notes: which letters may be made by sound, and which two letters carry a second, unrelated meaning as landing signals.

Three letters behave differently from the rest and are worth isolating before the table. R has no single-letter meaning in the Code, yet reappears in Chapter XII as distance. P means one thing in harbour and another at sea. And Z appears twice in Chapter XII — with one numeral and with four — for two entirely different purposes.

2. Chapter XI — Single-letter Signals

2.1 The A–Z table

The letters marked with an asterisk are those whose transmission by sound is restricted by Note 1 at the foot of the chapter (2.2 below). The three fishing-vessel meanings and the harbour/sea split of P are reproduced in the table and expanded in 2.3.

2.2 Signals marked with an asterisk — sound signalling under COLREG

Ten letters carry the asterisk: B, C, D, E, G, H, I, S, T and Z. The mark does not restrict the signal itself; it restricts one method of making it.

The note points the whole restriction at two rules of the 1972 Collision Regulations — Rule 34 and Rule 35 — without reproducing either: by sound, a marked letter is only lawful if what leaves the whistle also complies with those rules. The Code states no reason for the restriction and describes neither rule.

The concession at the end of the note is narrow and worth reading twice: it keeps the sound use of G and Z alive for fishing vessels fishing in close proximity to other fishing vessels. The fishing meanings of 2.3 travel under their own, separately worded condition — "when made by fishing vessels operating in close proximity on the fishing grounds" — and the Code does not state which of the two meanings the accepted sound signal carries.

2.3 Special meanings for fishing vessels

Three letters take on a fishing meaning, but not under the same condition. G and Z change "when made by fishing vessels operating in close proximity on the fishing grounds"; P at sea changes simply when "used by fishing vessels", with no proximity requirement in the text. The shift is not a variant of the general meaning — it replaces it.

2.4 K and S as landing signals

The second note of the chapter carves out a use that has nothing to do with the table.

In that setting K and S are not "I wish to communicate with you" and "I am operating astern propulsion": they belong to the set of landing signals for small boats with crews or persons in distress. The appendix that holds them sits outside the syllabus recorte for this unit, but the note itself is part of Chapter XI and travels with the table.

3. Chapter XII — Single-letter Signals with Complements

3.1 The twelve complemented signals

A complemented signal is a letter plus numerals, and the Code states for each letter how many numerals it takes — sometimes an exact count (A: three; L, T and Z: four), sometimes a closed choice (D: two, four or six; G: four or five), sometimes an open one (R, S and V: one or more). The letter names what is being reported; the numerals carry the value. The Code prints the set twice — once ordered by signal, once ordered by meaning — and the count of numerals is part of the signal, not a detail of formatting.

The Code marks Complements Table I with an asterisk in both listings and resolves it in a footnote: "* See page 121."

The pairing of speed is the second trap: S is speed in knots and V is speed in kilometres per hour. Both take one or more numerals, so the number itself gives no clue as to which unit was meant.

3.2 How the numerals are read

Four of the twelve signals split their numerals between two fields, and three of them split the same way — the first two numerals for the larger unit. Longitude is the single inversion: it counts from the back. Reading the split backwards turns a position into another position, which is why the Code spells the rule out inside the meaning itself.

Date is the widest field of the set: D takes two, four or six numerals. Chapter XII prints the three options without saying how to choose among them — the rule that assigns day, then month, then year to each pair of figures belongs to the general instructions of Chapter IV, not here. K and Z with one numeral are the two signals whose numeral is not a quantity: K's numeral points into Complements Table I to name the method by which the sender wishes to communicate, and Z's names the shore visual signal station being called, a Numeral to be approved by the local port authority.