Way back in 1836, Samuel F. B. Morse, along with Joseph Henry and Alfred Vail, invented an electrical telegraph system. Before telephones were invented, it could send messages over long distances by using pulses of electricity to signal a machine to make marks on a moving paper tape.
His version included only numbers. Vail soon expanded it to include letters and a few special characters, such as punctuation marks.
If you wonder how they decided which combination of signals was assigned to each letter, they studied how often each letter in the English language was used.
The most used letters were given the shorter sequences of dots and dashes. For example, the most commonly used letter in the English language — E — is represented by a single dot.
The original telegraph machines made a clicking noise as they marked the moving paper tape. The paper tape eventually became unnecessary.
Telegraph operators soon learned that they could translate the clicks directly into dots and dashes. Later, operators were trained in Morse code by studying it as a language that was heard rather than read from a page.
Today, it's possible to transmit messages in Morse code in any way that dots and dashes can be communicated. This includes sounds and lights, as well as printed dots and dashes.
Morse code was critical for communication during World War II. It was also used as an international standard for communication at sea until 1999, when it was replaced by the Global Maritime Distress Safety System. The new system takes advantage of advances in technology, such as satellite communication.
Today, Morse code remains popular with amateur radio operators around the world. It is also commonly used for emergency signals. It can be sent in a variety of ways with improvised devices that can be switched easily on and off, such as flashlights.
The international Morse code distress signal ( · · · — — — · · · ) was first used by the German government in 1905 and became the standard distress signal around the world just a few years later. The repeated pattern of three dots followed by three dashes was easy to remember and chosen for its simplicity.
In Morse code, three dots form the letter S and three dashes form the letter O, so SOS became a shorthand way to remember the sequence of the code. Later, SOS was associated with certain phrases, such as “save our ship" and “save our souls."
These were just easy ways to remember SOS, though. The letters themselves have no such inherent meaning.