There has been a discussion running on the Phoenix forum about flare and deviation in hooves. Its a subject that recurs fairly frequently and usually begins with someone asking how on earth you tell the difference between them.
Trimmers, other equine professionals and (of course!) owners often focus on flare (or deviation, or whatever you want to call it), not least because its visible. This is particularly so when a horse first comes out of shoes but you also frequently hear statements like "Oh yes, he needs to be trimmed every 6 weeks to keep the flare under control".
I've posted a lot about deviations, odd-looking feet, symmetry and so on over the years and its because the less I do to feet, the more interested I am in what feet do for themselves. Very often, horses know better than humans how their feet should work. We are typically more concerned with appearance but horses are only concerned with function and what works best to enable them to be as sound as possible.
In the past I've defined flare as being stretched white line and deviation as being a supportive hoof wall adaptation. That's ok as far as it goes, but what about a practical way to tell what you can safely trim and what you can't?
Think of it this way...In this photo, the hands of the adult are a vital support. Without them, little Jimmy (or whoever he is!) would fall flat on his face and there would no doubt be tears and trauma. In time he won't need them, of course, but for now the extra hands are essential.
Another photo, another adult holding a child's hand - but the purpose is quite different. Take away the adult's hand in this photo and the child would continue walking along quite happily. The hand-holding isn't supportive at all.
This to me is the difference between flare and deviation. But here's the thing...The difference doesn't really matter!
Of course, it does matter if you are proposing to take the hands away. In one scenario it would be a disaster. Similarly, if you are going to trim a horse, you'd better be pretty sure that you are taking off non-supportive flare rather than supportive deviation; if you get it wrong you will end up with a sore, less-sound horse.
But the real beauty of flare/deviation and the reason it doesn't matter which your horse has (provided you don't want to trim it off!) is that it won't matter to the horse either - as long as you leave well alone and leave the decision to him!
We always used to be told that leaving flare in place would lever away at better-connected hoof capsule but that just doesn't happen because the flare is stretched white line - its weak and compromised and is no match for healthy, well-attached laminae.
deviation is an essential support. If you trim it away it will rapidly re-appear unless and until its no longer needed to assist in the limb's movement and load-bearing - and while the horse is without it, he will be less comfortable and less capable, especially on tougher surfaces. So why would you want to take it away?
If you were the adult, and you weren't sure why the child was holding your hand, you wouldn't simply pull it away, would you? Don't you think it should be the child - who knows what he needs - who makes the decision whether to hold hands?