What a triangle vent is
A triangle gable vent fills the peak itself. The two sloping sides follow the roof, and the bottom is level. It is not a small accent floating in the triangle. It is the triangle, stopped short of the rake so the frame and the roof trim can both show.
These vents are molded high-density polyurethane, factory-primed white, and ready for paint. The material does not rot, and it cuts and fastens much like wood trim. A long triangle is still a made-to-order molding, so the pitch has to be right before it is ordered.
Where it looks right
Use it when you want the vent to disappear into the roof shape: cottages, farmhouses, and any gable where a circle would leave awkward triangles of siding on both sides. It is the wrong shape if you only have a small spot above a window and the roof pitch is not part of the design.
The bottom edge should sit level, and the sloping sides should run parallel to the roof, not steeper and not flatter. If they do not, the vent was made for a different pitch.
Open or closed
open triangle gable vents pass air along the whole peak. closed triangle gable vents follow the same roof lines and do not move air. A closed triangle is a finish piece for a gable that should look vented and is not. An open triangle is a large opening, so it belongs in a real ventilation plan, not as a guess. The sizing page explains net free area in words and tells you to confirm it on the product page.
A short sizing note
Measure the roof pitch first. Triangle vents are made for a specific pitch. LOT10829, a 108 by 29 inch triangle gable vent for a 6/12 pitch is the example shown here: a 6/12 pitch, 108 inches wide by 29 inches high. A different pitch needs a different vent, even if the width looks close. The one-fifth appearance rule is less useful here, because the roof geometry sets the height once the width and the pitch are chosen.
Pair it with
Pair a triangle with matching rake trim and a quiet wall. Adding a second round inside the triangle usually fights it. If the pitch is unusual, or you are not sure how to read rise and run, settle that before you order. The install overview covers laying out an open vent so the frame laps the cut.