Most people are familiar with reversible overcoats; by turning them insight out, one gets two garments for the price of one. The logological equivalent of this is the reversible word square, a square array of letters that contains words when read in normal left-to-right and top-to-bottom fashion (as in a crossword puzzle), but also contains words when read right-to-left and bottom-to-top.
Eckler, A. Ross
"Reversible Word Squares,"
Word Ways: Vol. 14
, Article 6.
Available at: http://digitalcommons.butler.edu/wordways/vol14/iss4/6