15 letters (pentagon with three letters per side)
ugy ner jlb dkw ios
unjudgelike , eyebrows
22 letters (hendecagon with two letters per side)
cb ai nt ew ok rg fm lh ds yp ux
cabinetworking , formaldehydesulphoxylic
My program makes 26-bit masks of the letters from each word, and then compares all mask pairs with bitwise or
operation to find the pairs of masks that combine to a "pair mask" with 12 bits set.
Then it searches through each of the words that belong to each of those pairs of masks, to find how many have
start/end letters in common.
If there is only one, then that's a great candidate to build a letter box. It's fairly trivial to build the Letter Box, given two
words that I know are unique in covering the 12 letters.
If there is more than one word pair for the 12 letters, it would surely be possible to build a Letter Box that only allows
one pair to work. I didn't follow that idea through.
The program outputs a list of all the unique word pair candidates. There are 250,170 candidate unique word pairs, for
twelve letters. I tried to pick relatively "ordinary" words for my solutions, as far as possible, because even though I am a
native English speaker, most of the words in the list are really strange to me!
It was simple to change the program to work with 15 letters or more. 22 letters was the most it could find a unique pair