A **signal flag** is one of a set of flags forming an established code for communication between vessels or between ship and shore. The International Code of Signals comprises twenty-six alphabetical flags, ten numeral pennants, three substitute pennants and the answering pendant. Each flag bears a fixed meaning when flown alone and combines with others to spell letter groups with codified meanings.
**Example**: The signal flag for the letter A in the International Code of Signals — white and blue, swallow-tailed — flown alone means ‘I have a diver down; keep well clear at slow speed’.
**See also**: [pennant](/glossary/pennant/), [q-flag](/glossary/q-flag/), [hoisting](/glossary/hoisting/), [ensign-at-sea](/glossary/ensign-at-sea/)