Phryctoria (Greek: φρυκτωρία) was a means of communication used in Ancient Greece. The method used two sets of five torches. When a signal needed to be sent, the appropriate torches were lit and placed atop towers called phryctoriae, and would be visible to the next tower (usually 20 miles away), and then relayed to the next hill in succession.
It was invented by the Greek engineers Cleoxenes and Democletus in the 2nd century BC. The coding system was as follows:
α β γ δ ε
ζ η θ ι κ
λ μ ν ξ ο
π ρ σ τ υ
φ χ ψ ω
When they wanted to send the letter o (omicron), they opened five torches on the right set and three torches on the left set.