Alfred Vail in the context of Relay

Morse code is a telecommunications method which encodes text characters as standardized sequences of two different signal durations, called dots and dashes, or dits and dahs. It is named after Samuel Morse, one of several developers of the system. Morse's preliminary proposal for a telegraph code was replaced by an alphabet-based code developed by Alfred Vail, the engineer working with Morse. Vail's version was used for commercial telegraphy in North America. Friedrich Gerke simplified Vail's code to produce the code adopted in Europe, and most of the alphabetic part of the (ITU) "Morse" is copied from Gerke's revision.

The ITU International Morse code encodes the 26 basic Latin letters A to Z, one accented Latin letter (É), the Indo-Arabic numerals 0 to 9, and some punctuation and messaging procedural signals (prosigns). There is no distinction between upper and lower case letters. Each code symbol is formed by a sequence of dits and dahs. The dit duration can vary for signal clarity and operator skill, but for any one message, once the rhythm is established, a half-beat is the basic unit of time measurement. The duration of a dah is three times the duration of a dit. Each dit or dah within an encoded character is followed by a period of signal absence, called a space, equal to the dit duration. The letters of a word are separated by a space of duration equal to three dits, and words are separated by a space equal to seven dits.

A telegraph sounder is an antique electromechanical device used as a receiver on electrical telegraph lines during the 19th century. It was invented by Alfred Vail after 1850 to replace the previous receiving device, the cumbersome Morse register and was the first practical application of the electromagnet. When a telegraph message comes in it produces an audible "clicking" sound representing the short and long keypresses – "dots" and "dashes" – which are used to represent text characters in Morse code. A telegraph operator would translate the sounds into characters representing the telegraph message.

Telegraph networks, used from the 1850s to the 1970s to transmit text messages long distances, transmitted information by pulses of current of two different lengths, called "dots" and "dashes" which spelled out text messages in Morse code. A telegraph operator at the sending end of the line would create the message by tapping on a switch called a telegraph key, which rapidly connects and breaks the circuit to a battery, sending pulses of current down the line.

American Morse Code — also known as Railroad Morse — is the latter-day name for the original version of the Morse Code, developed in the mid-1840s by Samuel Morse and Alfred Vail for their electric telegraph. The "American" qualifier was added because, after most of the rest of the world adopted "International Morse Code," the companies that continued to use the original Morse Code were mainly located in the United States. American Morse is now nearly extinct—it is most frequently seen in American railroad museums and American Civil War reenactments—and "Morse Code" today virtually always means the International Morse which supplanted American Morse.