Double jointed bits are better, preferably with a loose ring IMO. Single joints can pinch the tongue and straight bars can rest on the tongue both lead to resistance and opening of the mouth.
Better a loose ring fulmer snaffle with keepers for starters, but before mine got to the stage of bits for backing, they had the rubber small ring "baby" bits you use on in hand showing bridles and the Stallion had a straight bar stallion bit.
I d certainly want to get them used to the idea of a bit long before even starting backing.
Depends on the mouth conformation but I do quite a lot of them in Cambridge snaffles to start with or double jointed ported training snaffles, which I love and they seem to as well
At work we start all ours with a happy mouth straight bar and then usally got to and egg butt snaffle, sometimes fulmer or with cheeks. but if they dont get on with the egg butt snaffle we change it to a straight bar eggbutt.
Sorry, I should have said - my eggbut snaffle has a French Link mouth piece.
Personally, I don't like loose ringed bits. Always worried they'll pinch, and I find having to use rubber roundings a pain. They often chafe too, as my horses are generally fine skinned. The rings on my eggbut are large and reduce the likeliehood of the bit pulling through the mouth.
He's been bitted in one of those small black, straight-bar, showing snaffles, as a yearling when shown, but he's not been bitted since.
He's a TBxWB; very breedy and refined looking, and his tongue isn't large or fleshy. He's pretty good natured but is tall. Too tall for me really, so I am aware he needs to be accommodating about being bitted and I want the experience to go smoothly!! If he starts sticking his nose in the air, I wont stand a chance
Iv always used a losenger loose ring on the babies. Started with eggbutt's but i find they lean on it more so all mine are now in lose rings whether babies or not. They do not rub if they are the correct size and enough clearence at the sides of the mouth.
I have 2 exrace's in the loose rings and have hunted in them and not a scratch on their mouths at the end.
Interesting what you say about them leaning on the eggbut. Might have to consider that. He's a big lad and I'd not want to encourage that from the start.
We started ours in loose ring with lozenge. However she seemed much happier in her showing bit when she wore that, so we switched to a straight bar happy mouth, which she goes much better in.
Next year when she is 4 and the schooling begins then i will try her back in a jointed bit.