The Chappe optical telegraph was the world’s first practical telecommunication network, built across Revolutionary France from the 1790s. Tall towers carried a movable crossbar and two jointed arms; operators posed the arms into distinct shapes that relayed messages from hilltop to hilltop far faster than a galloping courier. This tool maps text to Chappe-style arm codes so you can picture how a message looked in motion. Everything runs in your browser.
How it works
A Chappe tower has a central regulator (the long crossbar) and two shorter indicator arms at its ends. The regulator could sit in a few orientations and each indicator could lock into seven angles, giving 196 possible shapes, of which roughly 92 made up the working code. Operators looked up message words and letters in a printed code book.
This encoder assigns each letter a position number in a simplified Chappe alphabet and derives illustrative arm angles from that number. It is a faithful teaching model of the principle — a one shape per letter mapping — rather than a reproduction of any single surviving code book, which used word-level codes for speed.
Tips and example
Encoding the word PARIS yields one Chappe code per letter, each shown with its regulator orientation and two indicator angles. Reading the arms is positional: the regulator sets a broad category and the two indicators refine it to a single code.
The real strength of the system was speed of relay: a short message could cross hundreds of kilometres in minutes on a clear day. Its weakness — needing daylight, good weather, and a staffed chain of towers — is exactly why the electrical telegraph soon replaced it.