What is "jetan"

thumb|upright=1.0|right|The 1963 Ballantine Books paperback edition of The Chessmen of Mars, showing a live version of Jetan being played in the city of Manator. Cover illustration by Robert K. Abbett.

Jetan, also known as Martian Chess, is a chess variant with unclear rules. It was created by Edgar Rice Burroughs as a game played on Barsoom, his fictional version of Mars. The game was introduced in The Chessmen of Mars, the fifth book in the Barsoom series. Its rules are described in Chapter 2 and in the Appendix of the book.