A1Z26
A is 1, B is 2, Z is 26.
Where it came from
A1Z26 is the most naive cipher imaginable: replace each letter with its position in the alphabet. Its origins are unclear because it is "obvious," but it thrives in puzzles, escape rooms and alternate-reality games.
It is never used for secrecy — a space separated by slashes is the only convention. Its charm is being solvable by a glance.
How it works
A→1, B→2, … Z→26; a slash or space separates words. That is the entire system. Because there is no key, it offers no security at all.
UWC encodes and decodes A1Z26 in your browser — handy when a puzzle hands you "8 5 12 12 15" and you want "HELLO".