Single (or center) vent
Most commonly found on American suits, the center vent was developed to keep the jacket sitting straight when a man was on horseback. It’s the default in American tailoring, though the European-style double vent is gaining ground.
Double (or side) vent
If your suit comes with a “made in Italy” (or “made in the UK”) tag, there’s a high likelihood it’ll have side vents, which afford a range of movement without exposing one’s rear to the world. The style also allows a suit to be cut closer to the body. Just make sure the skirt of your jacket lies flat to your body — if it’s flaring, you’re going too far.
Though favored by style icon of all style icons Cary Grant — who looked plenty dapper in an unvented suit jacket throughout North By Northwest — unvented suits haven’t been in wide usage since around 1993, and even then they felt a bit questionable. Our advice: appreciate the look on Mr. Grant, but leave it at that. We’ll let you know if this changes, but for now you’re better off sticking to either of the first two options.