What is the Two-square cipher?

Two-square encrypts letters in pairs using two keyed 5 by 5 squares, with I and J sharing a square. In the horizontal version the squares sit side by side; in the vertical version one sits above the other. Each pair of letters forms the corners of a rectangle across the two squares, and the other two corners are the ciphertext. When both letters fall in the same row (horizontal) or column (vertical), the pair is handled specially, which can leave part of the message readable.

Enter both keywords and choose the layout. If a decode is close but scrambled in pairs, try the other layout.