A simple code table gives each of about eight messages one four-pulse pattern of short (S) and long (L) signals, and the same pattern is used on a horn, a lamp, a mirror or a drum. Using only patterns with an even number of long pulses means any single wrong pulse gives a pattern that is not in the table, so the receiver asks for a repeat instead of acting on a wrong message.
Part of Signal Systems and Communication
A code table is the single list of patterns and meanings that every post and signalling channel in a settlement uses.
A Table That Follows The Rules
The design rules are in Code Systems. Below is a table built on them. S is a short signal and L a long one; a gap of about three short-signal lengths separates repetitions. It is an example to adapt, not a standard: change the assignments, keep the rules.
| Message | Four-pulse pattern | Flag by day | When used |
|---|---|---|---|
| ALARM: danger now, execute the plan | S S S S, repeated | Red flag waved | Confirmed threat or fire |
| RALLY: everyone to the assembly point | L L L L, repeated | Dark flag waved | Any order to assemble |
| DANGER: confirmed threat, all posts alert | S L L S | Red flag raised | A threat seen but not yet close |
| ALERT: attention, stand by | L L S S | Dark flag raised | Before a message follows |
| VISITORS: strangers approaching, no hostile sign | S L S L | Yellow flag raised | Strangers sighted |
| ALL CLEAR: normal operations | L S L S | Green flag raised | After any alarm |
| ACKNOWLEDGED: message received | S S L L | Flag dipped once | After every message |
| NEGATIVE: no, or do not proceed | L S S L | Flag waved side to side | Answer to a question |
All eight patterns have four pulses and an even number of longs (0, 2 or 4), so any two of them differ in at least two positions. Of the 28 pairs (8 x 7 / 2), none differs in fewer than two places, and none begins another because all have four pulses.
What Changed From The Parent’s Table
- ALERT and ALL CLEAR no longer share a beginning. In the parent table one long blast, ALERT, is also the start of ALL CLEAR (1 long + 2 short), VISITORS (2 long) and RALLY (3 long), and one short blast, NEGATIVE, is the start of ACKNOWLEDGED (2 short).
- Three alarm patterns became two. The parent horn table has “repeated short blasts”, “3 rapid blasts” and “continuous blasting” for similar urgency; this table has ALARM and DANGER only, one for execute-the-plan and one for alert-but-not-yet.
- Continuous blasting is gone. An operator cannot blow or beat without end; the alarm is a repeated pattern.
- The all-clear flag is no longer white. A white flag is widely read as truce or surrender (recalled), and an approaching stranger will read it so; use a colour with no existing custom.
Whistles: A Short Table For Posts And Patrols
Whistles are short-range and used by sentries who are out of breath, so keep them to counts, not lengths.
| Whistle | Meaning |
|---|---|
| 1 blast | Attention or halt |
| 2 blasts | Acknowledged |
| 3 blasts | Danger, return or take cover |
| 1 long blast | Come to me |
Posts and patrols use this single table. Watch and Patrol Scheduling gives posts and patrols different meanings for two blasts, which is the collision the single table removes.
Method: Adopting A Table
Step 1: Choose the patterns, not the order of the list. Put the alarm on the simplest pattern.
Step 2: Print and pin it at every post, with a version number and a date.
Step 3: Drill each message in rotation. Each person should send and read every message at least once in a drill month; the parent article sets weekly drills and monthly night drills.
Step 4: Change the table after any compromise, as the parent article requires.
Common Mistakes
- Adding new messages in the field. A message that is not in the table is not a message.
- A pattern read by length alone. A horn long enough to be “long” in the open may sound short in the wind. Practise.
- Different tables on different channels. A person who hears the horn and sees the flag must get the same message.
What’s Next
- Learn next: Priority and Urgency Codes — what to do when two signals arrive together
- Combine with: Authentication and Security — knowing the signal is yours