These guys are going on about their bespoke clothing, like that's any help for the kind of novice who timidly asks if he would be too bold or out of line in a ventless jacket. If you have a good bespoke tailor, any kind of vent will look good on you. Here are some other pros and cons regarding ready-to-wear (RTW) jackets for the novice to consider. Single vent and ventless are easier to alter in the body. The average alterations tailor cannot take in the body much with a double-vented jacket since it can require rebuilding the vents. If you have a slight athletic build (say, an eight-inch drop), you might avoid double-vents since your average alterations tailor cannot bring in the waist for you. Also, the single vent will not lay flat - either one side stands out from the body, or the two sides open like an inverted V - if the jacket does not fit well. RTW jackets do not often fit many people well. The open vent can be caused by a balance problem, i.e., either the back is too short or too long as, e.g., your posture is too stooped or too erect. It can also be caused by asymmetry such as a dropped shoulder. Also, the lighter in weight the fabric, the more likely you will have odd ripples and an ugly vent. Single vents will lay flat in RTW jackets in very heavy fabrics (e.g., tweeds) more often than they will in RTW jackets in light fabrcis (e.g., the dread "all season wool" or cheap "super 150s"). Since most RTW comes now in very light fabrics, the odds of a clean back and a well-behaved single vent are not so great. The good news is you can close the vent for about $20-$30 and, while this does not make the coat fit any better, it does remove the ugly vent. There's also the general look of the ventless vs. the double vent. The ventless as some have said tends to look more formal. The ventless can also be slimming compared to the double vent, which adds bulk to your sides. So a double vent might look better on a beanpole who wishes to bulk up, and a ventless jacket might look better on a short man who wishes to look slimmer and thus taller. A double vent on a short man may help to make him look like a cannonball, and a ventless jacket on a tall man may make him look more like a telephone pole. I read a book that went on and on about this once. I think the author was Chesterfield. Anyway, these tendencies are not sure things, but something to consider with other characteristics of a suit, such as stripes to add height, dark colors to look more slim (and tall), heavy fabrics to add bulk, and horizontal stripes for criminal.