Have you ever worn a suit, and you think it should fit good. It’s your size, the arms feel good, the lines seem right, but when you put it on, it just doesn’t fall right? Maybe it’s too snug, perhaps a bit too loose? Chances are, if you can’t figure it out, it’s because of the vents in the back.
If you are a bigger guy, I’m going to suggest you get the side vents. It usually suggests a penchant for more European styles, but I’d like to stress that even if you aren’t a big guy, don’t be scared off the side-vent. You can still be slim, wearing a slim-fit suit, with side vents and look good.