A 6×6 Grill for Secret Writing
A grill consists of a square of paper in which square holes have been cut in a certain way. In this case, the paper consists of a square with nine smaller squares cut out. Place the square on a piece of paper and write the first nine letters of some text in the holes. Then, turn the grill a quarter turn and write the next nine letters. Continue the procedure for two more turns. The given text consists of words in English, Spanish or Esperanto.
 H. Lysing (J. L. Nanovic), Secret Writing: An Introduction to Cryptograms, Ciphers, and Codes, New York: Dover Publications, 1974 pp. 36–37.
 Strømmen. "1000 Most Common Spanish Words-Frequency Vocabulary." (Apr 24, 2023) strommeninc.com/1000-most-common-spanish-words-frequency-vocabulary.
 1000 Most Common Words. "1000 Most Common Esperanto Words." (Apr 24, 2023) 1000mostcommonwords.com/1000-most-common-esperanto-words.