In my opinion, in order of severity, mildest to harshest: Part One: for direct reining: -flat-nosed sidepull/jumping hackamore:
function is that of a halter with reins attached, but purpose-built. Pressure on bridge of nose. -scawbrig:
as described by an earlier poster, the strap beneath the jowl tightens. Pressure on bridge of nose and curb area. -lightrider:
much like a scawbrig, but more modern design. Doesn't slip around. Pressure on bridge of nose and curb area. -indian bosal:
functions similarily to the scawbrig/light-rider, but instead of a single strap under the jaw, it is two straps crossing over one another under the jaw. Often made of rope. Release can be slow. Pressure on bridge of nose and curb area. Please use one only with metal rings for the cross-under straps to cross through, as pictured. This allows for less friction and a better release. -cross-under:
made popular by Dr. Cook. Two straps coming from the crownpiece cross under the jaw and exit at rings at the sides of the noseband. Release can be slow. Pressure on bridge of nose, middle
of jaw area, cheeks and poll. -equi-bridle:
odd pulley-style bridle. When reins are pulled, a sliding pulley attached to a curbchain moves up the cheekpieces, putting pressure on the bridge of nose, curb area and poll. Never have tried this one personally. -LG zaum bridle:
(a variation of this is the flower hackamore) similar to a standard hackamore, but with much less leverage. My personal favourite of all bitless designs. Allows a good, direct pressure with a bit of lift. Pressure is put on bridge of nose, curb area and poll. -hackamore:
this has the most leverage of all bitless options, and should be largely treated as a curb bit. Can be direct-reined in, but signals aren't very clear and need reinforcement from seat and legs. Pressure is put on the bridge of nose, curb area and poll. Part Two: For Neckreining: Bosal:
the bosal works off of pressure on the bridge of nose, and the jawbones. Most definitely cannot be used for direct-reining. Little S:
similar to a hackamore, this bitless option can be used for direction in direct reining, but is really only meant to be used neckreining. Very mild, this bridle puts pressure on the bridge of nose and curb area. Very little leverage. Please use one with a flat noseband, as pictured.
Please note that none of these images belong to me.
I hope there were no omissions in this post; and I can go in to further detail about any bridle on request.
Best of luck on your bitless journey!