Morse code is an alphabetic code of long and short sounds, originally transmitted by telegraph. Each letter in the alphabet has a corresponding sound or series of sounds unique to it. The long sounds are referred to as dashes, while the short sounds are dots. Varying lengths of silence denote spaces between letters or words.

To make a dot on a telegraph, the telegraph key or switch was depressed and allowed to rapidly spring back. To make a dash the key was held down longer before allowing it to rebound. Thus messages were sent by tapping the key in a rhythm of coded letters. Messages were received via a radio transceiver, sounding like dots and dahses of static.

American Samuel Finely Breese Morse (1791-1872) invented the telegraph and Morse code in 1836. The code was successfully tested on 24 May 1844, when Morse himself sent the first message between Washington DC and Baltimore: "What has God wrought?"

The most well known Morse code phrase is SOS or save our souls. SOS was chosen because the code for it -- 3 dots followed by 3 dashes followed by 3 dots -- is unmistakable as anything else and recognizable even to those who do not know Morse code.

Before SOS, the code was CQ which meant anyone listening, please respond. A third letter followed, revealing the reason for the hail. In the case of distress, it was a "D." So it was that when the Titanic hit an iceberg shortly before midnight on its maiden voyage in April 1912, operator John G. Phillips sent a mayday message using the old emergency code and the new one. Titanic's exact Morse code transmission that cold night was, CQD CQD SOS SOS CQD DE MGY MGY. MGY were Titanic's call letters, while DE meant from. An innocuous looking message that literally translated to:

All hail our distress!
All hail our distress!
Save our souls!
Save our souls!
All hail our distress!
From Titanic!

The California was less than 20 miles (32km) away and had enough boats to save everyone aboard Titanic, but their radio officer was off duty because it was the middle of the night. Titanic tried to get their attention by firing rockets. On duty officers aboard the California watched the rockets but failed to understand. The next morning when the California's radio operator resumed duty, they learned from other ships what had happened. The Carpathia did respond to Titanic's Morse code distress call immediately, but that ship was 58 miles (93km) away. By the time Carpathia arrived, it was too late for more than 1500 of Titanic's passengers.

Because of Titanic's disaster it became law that a ship must always have a radio operator on duty.

Telegraph operators created shorthand that endures today in completely unrelated settings. One example is the use of "30" by reporters to mark the end of their copy. This was Morse code for I have no more to send.

Morse code is still used today by the Maritime, Military and Amateur Radio Services. Morse code can also be sent by light, using short or long flashes to denote dots and dashes.