On a suit jacket a vent is the vertical slit that can be found on some jackets in their lower rear portion. Vents are designed to make sitting while wearing a suit jacket more comfortable; though were originally created to make riding horses easier. Another advantage to having jacket fence is the fact that wrinkles are prevented as the suit coat is allowed to comfortably expand when the wearer sits down. Without vents it is quite common to see wrinkles develop in the lower third portion of the suit over the course of a day.
Today suit jackets can be classified into three options in terms of their venting style; without a vent, having one vent in the center, and having two vents (one on either side of the coats midpoint).
Venting adds cost to the production of a suit which is why it is more common to see single vented suits rather than double vented suits. Larger men should be cautioned to not forgo vents in an effort to save money as tailored vents are essential for comfort and style on a suit that is destined to be worn on a larger frame.