Statjunk is correct in his basic description of a barrel rib. It's intent is to give the shooter a flat plane to look or sight dowm, instead of looking at a drop and percepted rise in the appearance of the barrel when aiming. The ventilated rib is usually set on individual posts soldered to the barrel, and then the flat rib itself is attached to these posts. A solid rib is just that....a solid raised rib on the barrel with no airspace underneath it. No rib is simply a plain barrel. Hope this helps................