What is the Four-square cipher?

Four-square uses four 5 by 5 grids. Two hold the normal alphabet and two are scrambled with keywords. Each pair of letters is found in the normal grids, and the letters at the other corners of their rectangle, in the keyed grids, become the ciphertext. Since a 5 by 5 grid only fits 25 letters, some versions merge I and J while others leave out Q. If one gives nonsense, try the other.

Example

FYGMKYHOBXMFKKKIMD with the keywords EXAMPLE and KEYWORD, leaving out Q, decodes to HELPMEOBIWANKENOBI.