Put a shotgun, or fusil's buttplate flat against a floor, next to a perpendicular wall, or trim. The distance from the wall to the front sight is the " Pitch". For most guns, today, pitch is negative, that is the muzzle is further from the wall than the heel of the stock, and rear of the action.
Barrel chested men, and most women, need to have some down-pitch so that the butt plate doesn't Dig into the upper chest muscles. Flat chested men and women need less down pitch.
If you stand the gun against a wall, and the muzzle touches the wall before the heel of the stock does, such a gun will often slip down on your arm or chest when its fired, and some will rotate upward at the wrist, smacking your face with the comb of the stock. It depends largely on how you hold a shotgun to your body, and where you position your face in relation to the comb.
Choosing a pitch angle for a gun is often decided by the kind of shooting you intend to do with it. Duck and Geese hunters, for instance, do a lot of shooting above their heads. The stocks need to be straighter, and often shorter than what would be comfortable if they were choosing a shotgun to hunt flushing birds, like quail, or grouse, or Pheasant, or partridge. Some small bird hunters are walking ridges, and shooting down into ravines, and swales to get birds. Their guns need different LOP, and pitch angles to be comfortable.
For the most part, American shooters just MAKE DO with what the commercial companies put on guns. This is quite different that the approach taken by European hunters, who shoot custom fitted guns as the norm. I think most MLERs just " make do", with their smoothbores, too.
