A type of diagram invented by Lewis Carroll (the name is an abbreviation of "Lewis") that can be used to determine the number of minimal
covers of 
numbers with 
members.
Lew k-Gram
See also
Minimal CoverExplore with Wolfram|Alpha
References
Macula, A. J. "Lewis Carroll and the Enumeration of Minimal Covers." Math. Mag. 68, 269-274, 1995.Referenced on Wolfram|Alpha
Lew k-GramCite this as:
Weisstein, Eric W. "Lew k-Gram." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Lewk-Gram.html