The simple answer- a slight difference in the shanks.
The longer answer- I've had this discussion with people several times on the "tom thumb" threads. Basically, I see no major difference myself
I sort of put tom thumbs, argentine snaffles, jr. Cow horse, and all the other broken mouth curb bits in the same catergory of similar bits.
But other people say that the slight sweep to the shank makes all the difference in the severity of the bit. The tom thumb is too straight.
But some of the shank differences are so slight, I really think they are almost the same myself.
How much difference is there really in these? http://www.horseforum.com/attachment...omthumbbit.jpg http://www.tackranch.com/253120.jpg
The above are both called tom thumbs.
These below are all called something other than tom thumb: http://s7d5.scene7.com/is/image/PetsUnited/TE003480 http://flynnssaddleshop.com/wp-conte...011/06/881.jpg http://www.chicksaddlery.com/Merchan...0001/25526.JPG http://www.onestopequineshop.com/ass...ges/218009.jpg http://www.chicksaddlery.com/Merchan...001/636010.jpg http://www.horsetackco.com/images/240689.jpg http://upload.wikimedia.org/wikipedi...Pelham_bit.jpg
See, to me they are all variations on the same theme. Sure, some you can use with two reins, some have a straighter shank, some have more or less leverage, but basically they all function pretty similar in my eyes.
Really, I think the name "tom thumb" has become synonymous with cheap quality bits with straight-ish shanks. But there are a lot of other bits that are nearly the same that don't have the bad reputation. I think they have become some sort of scape-goat for everyone's bitting ills.
The way I see it, if you don't want to catch flack, just call it an Argentine snaffle.
(Donning my flame suit because I know everyone on the planet disagrees with me).
I'm not saying the tom thumb is the BEST choice for this style of bit. Almost every other bit I posted a link to is probably better made. All I'm saying is that I don't think the tom thumb deserves it's evil reputation. There are a whole lot of other bits out there much worse that fly under the radar. But the tom thumb is the one everyone loves to hate.