I do too many crosswords.
The New York Sun runs an excellent crossword, which yesterday was entitled "Typewriter Trivia" and featured the following four long entries, here with their clues:
Disposition to credulity (and the longest common word that alternates typing hands): ANTISKEPTICISM
Violet variety (and the longest common word that uses just the right typing hand): JOHNNYJUMPUP
Seesaw (and the longest common word that uses just the top typewriter row): TEETERTOTTER
Knitted garments for women (and the longest common word that uses just the left typing hand): SWEATERDRESSES
If you want to read more about crosswords, check out Amy Reynaldo's blog Diary of a Crossword Fiend -- the link is to the entry on the crosswords which were published yesterday. This particular entry also mentions her book How to Conquer the New York Times Crossword Puzzle: Tips, Tricks and Techniques to Master America's Favorite Puzzle, which comes out today.
Wikipedia, which has a lot of silly lists like this, tells us that TESSERADECADES and AFTERCATARACTS are also typeable entirely with the left hand, though they're less common. TETRASTEARATES also has this property, according to A Collection of Word Oddities and Trivia. To someone who does too many crosswords, it's also recognizable as a word that would often be found up against the right or bottom edge of a crossword, because it consists of letters that occur often at the end of words. (ASSERTS, ASSESSES, and so on are common in those positions.)
The longest word entirely typeable with the middle row is SHAKALSHAS, which is shorter than the others and also more obscure. There are no words which are entirely typeable with the bottom row of the standard keyboary, since it has no vowels. (The
What I'm led to wonder is, do we expect such words to be long? Longer words should be possible if we have larger sets of letters to work with. Georges Perec once wrote a novel called La Disparition which doesn't contain the letter "e" (and in French, no less, where this should be harder than in English!) and also Les Revenentes which contains no a, i, o, or u. The first of these was "translated" into English as A Void, although I don't know enough to know if this can really be called a "translation". This is considered noteworthy. But if someone wrote a book without a Q in it, nobody would care!
Letter frequencies can be found here, without regard for the frequency of the word in ordinary text. We can compute that 48.03% of letters come from the top row; 32.49% from the middle row; 19.74% from the bottom row. So we expect that it will be easiest to form words which come entirely from the top row, then the middle row, then the bottom row; this is in fact the case. Similarly, 61.27% of letters are drawn from the left hand; we can form even longer words with the left hand (14 letters) than with the top row (12 letters).
As for why we can form such long words with alternating hands? The Dvorak keyboard, which is a widely used alternate to the standard QWERTY keyboard, is set up so that letters which often occur consecutively are typed with opposite hands. It wouldn't surprise me if QWERTY uses that same principle, even if it's not consciously incorporated into the design.
(An earlier version of this post had some broken links. They should be fixed now.)