Before voice radio, a simple pattern meant distress. SOS is the Morse code distress signal (· · · — — — · · ·), formally adopted for maritime use, that indicates a ship needs immediate assistance.

The sequence was chosen for its clarity and ease of recognition, not as an abbreviation of a phrase. It can be sent by radio, sound, or light. Although Mayday has largely replaced SOS for voice distress, the Morse signal remains recognized and is still taught.

Visual SOS—three short, three long, three short flashes—can be sent with a lamp or other means when radio fails. The signal’s simplicity has given it a permanent place in maritime emergency procedure and in popular culture.

• Morse code distress signal (···–––···)
• Adopted for clarity in radio and visual signaling
• Still recognized alongside voice Mayday
• Can be sent by light, sound, or radio