Type your text and two keywords, and the tool encrypts it in letter pairs using two keyword squares and two plain squares.
Four 5×5 grids are arranged in a square: the top-left and bottom-right are plain alphabets, the top-right and bottom-left are keyword squares. Each plaintext pair is located in the two plain squares, and the ciphertext letters are read from the two keyword squares at the intersecting corners.
Four-square uses two separate keyword squares instead of one, and it never has to insert filler letters between doubled letters. Only an odd-length message is padded with a trailing X.
Yes. Encoding then decoding an even-length letter message with the same two keywords restores it exactly (J is merged into I, output uppercased).