Morse code is one of the most influential communication systems ever devised, a method of encoding text characters as sequences of two signal durations—short signals (dots, or "dits") and long signals (dashes, or "dahs")—that fundamentally transformed long-distance communication and shaped the modern world. The system is named after Samuel F.B. Morse, an American painter and inventor, though the code itself was substantially developed by his assistant Alfred Vail in the 1830s and 1840s.

The original American Morse code, used on the first telegraph line between Washington, D.C. and Baltimore in 1844, differed significantly from the International Morse code used today. American Morse included internal spaces within some characters and used dashes of varying lengths, making it complex and difficult to learn. In 1848, Friedrich Clemens Gerke created a simplified version for German telegraphs, which was later refined and adopted internationally at the International Telegraphy Congress of 1865. This International Morse code uses only two symbol lengths—dot and dash, with the dash defined as exactly three times the duration of a dot—making it cleaner and more systematic.

The timing structure of Morse code follows precise ratios that are essential for clear communication. A dot is the basic unit. A dash lasts three units. The gap between elements within a single character is one unit. The gap between characters is three units. The gap between words is seven units. These ratios ensure that even at high speeds, the rhythmic patterns of characters remain distinguishable. Speed is measured in Words Per Minute (WPM), calibrated using the word "PARIS" as the standard reference word because it contains 50 dot-units, meaning one WPM equals 50 units per minute.

The character assignments in International Morse code are not arbitrary. Morse and Vail studied letter frequency in English by counting type in a printer's type case, and they assigned shorter codes to more frequent letters. E, the most common letter in English, is a single dot. T, the second most common, is a single dash. A and I are two elements each. Longer, less frequent letters like Q (dash-dash-dot-dash) and Y (dash-dot-dash-dash) received four-element codes. This frequency-based assignment was an early application of what would later be formalized in information theory by Claude Shannon as variable-length encoding—the same principle behind Huffman coding and modern data compression.

Morse code's most famous signal is SOS (··· ––– ···), adopted as the international maritime distress signal in 1906. Contrary to popular belief, SOS does not stand for "Save Our Ship" or "Save Our Souls"—it was chosen purely because its Morse pattern (three dots, three dashes, three dots) is unmistakable and impossible to confuse with any other combination. The signal is sent as a continuous sequence without the normal inter-character gaps, creating a distinctive rhythm that even untrained listeners can recognize.

Despite being replaced by digital communication systems for most practical purposes, Morse code endures in several niches. Amateur radio operators worldwide continue to use Morse code (called CW, for "continuous wave") because it can be decoded at signal strengths far below what voice communication requires, making it invaluable in poor propagation conditions and emergency situations. Aviation navigational beacons (NDBs and VORs) still identify themselves in Morse code. Some military and intelligence applications maintain Morse code capability as a fallback communication method that requires no sophisticated equipment—a functioning Morse code transmitter can be built from basic electronic components.