What kind of books are "battledores" here refer to?

We must not omit to notice the Chap books specially intended for the children. These were of three classes, in the first of which we place the battledores, A B C's, primers, and others intended for educational purposes. The battledores consisted of a sheet of cardboard, usually bi-coloured, which folded once or twice, printed on both sides with the alphabet, the Lord's Prayer, and some spelling lessions, with a few wood-cuts, occasionally superior in execution.