Helen (ofTroy) suggested a discussion of gambling words, starting with horse racing.

Not that I know a good deal about it, as I go to the races infrequently, but I'd be a poor Marylander if I didn't know something about it and go to the track now and then. For openers:

Nag, screw: a worthless horse
Jock, rider, boy: jockey
Mudder: a horse that runs well in the rain or on a wet, sloppy track
Infield: the oval area inside the track
Tote board: the electronic parimutual board in the infield showing the details of the horses, the odds, and (after the race is over) the payoffs.
Bet across the board: a combination bet in multiples of 3 on the same horse for win, place and show.
Win: 1st place
Place: 2nd place
Show: 3rd place
Daily double: a combination bet on the winners of the (usually) 8th and 9th races
Exacta: a combination bet on what horses will win and place in any race
Trifecta: same as exacta, except on win, place and show in any given race
Morning line: the list of starters and the odds on each for all races at the track, as first published in the earliest editions of the newspaper or Racing Form

My Lord Baltimore - - No quibble intended, (Let him who has not typo-ed cast the first aspersion) but, to my proof-reading eye, "parimutuAl" looked funny. ILIU and, sure enough, it's spelled "parimutuEl" but, curiously, the entry states "mutuEl = mutual" Yet one more instance of the inconsistences of English orthography or is there another explanation? Is this peculiar to racing?

Nearly 30 years ago now, my Dad used to work Saturdays on an old manual tote board, putting up the divvies after each race, along with the scratchings, etc. The only difference I could see in your list was that what you call an exacta we in NZ call a quinella, don't ask me why. I love horse-racing, especiaaly trotting and pacing, but could never stand to throw my money away on it.

parimutuel
Dang, you're absolutely right -- I knew it was with 'e' and never noticed it. Since I have never seen the word used for anything else than a betting system at horse races, I don't know that it has any other use. I was under the impression it was invented by the American Totalizator Co. (or whatever it's called) in the 1920's. OED, anyone?

Webster's Unabridged suggests the answer:

Parimutuel is directly from the French, pari mutuel, meaning "mutual stake," whilst "mutual" is indirectly derived from the French, mutuel, through the Middle English "mutuall."