Problem: Using any 21 alphabetic letters of your choice, each one exactly five times, construct a group of 21 five-letter words with the properties that (1) any two of the 21 letters being used will be found in exactly one word, and (2) and two of the 21 words will have exactly one letter in common.
Eckler, A. Ross
"The Twenty-One Words,"
Word Ways: Vol. 8
, Article 17.
Available at: http://digitalcommons.butler.edu/wordways/vol8/iss4/17