The Playfair Cipher is a manual symmetric encryption technique that encrypts pairs of letters (digraphs) instead of single letters. Developed by Charles Wheatstone in 1854, it uses a 5x5 grid filled with a keyword or phrase, where each letter is placed in the grid, and the letter 'J' is typically combined with 'I' to fit the 25-letter format. To encrypt a message, the letters in each digraph are located in the grid. If they are in the same row, each letter is replaced by the letter to its immediate right; if in the same column, they are replaced by the letter directly below. If they form a rectangle, each letter is replaced by the letter in its own row but in the column of the other letter.