'And all but the most formal of a Sloane's suits (no vents) have two vents, for two reasons: he can put his hands in both pockets, and when he moves fast or dances, the skirts fly up, showing the silk lining (raspberry is a bit charlie, but it does look fine).'
The Official Sloane Ranger Handbook, Ann Barr & Peter York (1982)
'The double vent or side slit offers the best combination of function and form. When you out your hands in your trouser pockets, the side vent's flap stays down, covering the buttocks. If you are seated, the flap moves away, thereby minimising creasing. Most men shy away from side vents because getting them to fall correctly requires more attention. When fit properly, however, the jacket's two two slits trace the outside lines of the body. An illusion of height is also created, because the side vent moves the observer's eye up from the bottom of the jacket. Since double-vented coats are costlier to manufacture and more difficult to fit than other models, you see them less frequently. However, the well-designed side-vented jacket gives its wearer a dash of style that bespeaks its English pedigree and custom-tailored tradition.'
Style and the Man, Alan Flusser (1996)