If you are even considering this bit, read about how it really works and why it's one of the worst bits you can use: http://todayshorse.com/the-trouble-with-tom-thumb/
I use a Tom Thumb on my horse that would NEVER respond to a snaffle (believe me, I tried) and it works great.
I have used the tom thumb on all my horses,it never fails to lower the high heads,soften hard mouths,even emproved neckreining with green horses