Hi Kate. The thickness of the bit determines its severity, together with the riders' hands - a thicker mouthpiece is less severe than a thinner one. However, some horses are worried by a very thick bit & there are other factors to take into consideration. How responsive is your horse? How large are his teeth - the bit sits on the gums but, if it's so thick he cannot comfortably close his jaws you'll be in trouble. All too often we see nosebands used to keep the mouth closed, especially dangerous in younger animals who are still going through adult tooth changes. Kirk will be 3 years old next March & last month he lost a front milk tooth - something vets agree would have happened as a yearling. There are no hard & fast rules for development. To try & help you decide, what bit do you currently use? Is your horse responsive to that bit? If so, stay with a similar thickness.

Bit less bridles come in a variety of types, some ultra severe. My Apollo had been abused in his mouth. I tried everything but only got fingertip control using a straight bar western bit with a tongue port ( extra space for the tongue ) & a leather strap ( not a chain ) in the curb groove. This put pressure on his poll & jaw instead of on his damaged gums. However, I felt happier using an unvulcanited, straight bar, thick rubber snaffle. When I first tried to put it in his mouth he was very resistant, due to the thickness I think, but he quickly came to love that bit. I enjoyed his exuberance & was happy for him to express himself. On the roads he was a perfect gent. In a wide open space he would skip around to see if I fancied doing something more exciting. I apologise for raising more questions than answers but, hey, it's not a matter of either black or white - like many things. Cheers, Jo.

I am not a bit expert at all and Jo has much more experience than me but I feel I need to point out the contradiction in part of what Jo says. I agree that a thick bit in a small mouth is uncomfortable. So, conversely a thin bit in a large mouth clanking and banging around would be no good but that does not mean that it's the thickness of the bit that determines its severity it's the incompatibility of the bit to the horses mouth which causes the severity (thin or thick) as well as hands etc. as Jo rightly says - hence the contradiction.
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While there is of course a level of truth in that a thin bit can cause pain more easily I feel that the "thick is kind, thin is cruel" logic is a bit of a traditionally held myth that must be treated with caution.
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However, I strongly support what Jo is saying also that bitless is NOT by any means automatically kind again, it needs to be thought through.
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Kate - it's interesting that you give your horses height but I think it's more about his mouth and your hands than his height however, as I said, I am in no way an expert in this area and would be interested to see others views.

Does Monty do a french link version? Some horses aren't always keen on the nut cracker and prefer a lozenge but that may throw too many questions at you! I just bought a lovely sweet iron with copper logenze full cheek snaffle for my 4 year old as she's off cubbing and hunting a bit this season so she may well lean on her happy mouth when the field gallop off. She doesnt like a single jointed at all. She's 16hh hunter build with a 5.5' mouth. Her happy mouth is a thick bit straight bar snaffle and she doesnt struggle to close her mouth and in my head its nice on the corners of her mouth and not too cold on her tounge. The sweet iron is thinner however but only as thin as a normal bit you may well pick off the shelf at a general equine shop. probably 2cm thick at each edge? She seems happy in this too when she last went cubbing.
I have always been told and said myself with my young mare that at some point I'll move onto an 'adult' bit- a sweet iron/copper snaffle and leave the happy mouth behind as it wont get her light in the mouth and she'll lean on it when I start her flat work schooling and flexion work but...
I've been doing it for a few weeks now in her happy mouth and she flexes just brilliant. She respects the happy mouth and the thickness of the bit hasn't dulled her sensitivity so with the comfy happy mouth we will stay for our general work.
That in mind she will have her sweet iron for hunting.... we may need a tad more brakes ;)