In the year 200 B.C, Polybius, a Greek historian and scholar, invented the Polybius Square. This Square consisted of a grid filled with the Greek alphabets. The rows and columns of the grid were numbered and thus any alphabet could be referred to by its row & column numbers. To encrypt a message, each letter of the plain text was located in the grid and replaced by its corresponding row and column number. Thus this system essentially replaces letters with numbers, a first of its kind recorded in history. The efficacy of this system lies in the fact that these numbers could now be transmitted in various ways: using a drum, using smoke, using flash-lights, tapping the bars in prison cells, etc.
The Polybius Square in English could look like this: