I regularly hear the phrase 'in the saddle' but 'on the saddle' seems more appropriate as you are literally on top of it, (I'm using the original definition of literally there!).
I can sit in an armchair. A baby can sit in a high chair. I can sit in a car.
Up to this point, it seems that if the object you are sat in/on surrounds you in some way, then you must sit in it.
I can sit on a plane (at least I've never heard someone say "I'm sitting in a plane right now"!), which is weird because you're actually inside the plane!
But i can only sit on a sofa, no one ever sits in a sofa.
So what gives? Is it just the fact that one option sounds better than the other, or is there some deeper understanding of the English language that I am missing?