Puzzle: Using 13 different letters of the alphabet, each repeated 4 times, construct 13 4-letter words with the property that each possible pair of letters appears in only one word, and each possible pair of words has only one letter in common.
Keller, Robert Cass
"The Thirteen Words,"
Word Ways: Vol. 5
, Article 5.
Available at: http://digitalcommons.butler.edu/wordways/vol5/iss2/5