What a rectangle vent is
A rectangle gable vent is a square or rectangular louver with straight sides and level slats. It is the plain shape. That is a virtue on a Craftsman house, a modern farmhouse, or any gable where the windows are already rectangular and you do not want a second geometry.
For a given width and height, the straight outline also gives the slats a large clear field, which is why people choose it when airflow matters and the wall can take a simple opening. The vent itself is still molded high-density polyurethane, factory-primed white, ready for paint. It does not rot, and it cuts and fastens much like wood trim.
Where it looks right
Center a square high under the peak, or set a wider rectangle across a low gable the way you would set a transom. Craftsman, farmhouse, and simple colonial walls all accept it. It can sit above a pair of windows if the width relates to the window group, not if it is floating at a random size.
Keep the sides plumb and the head level. A rectangle shows an out-of-level install faster than a circle does.
Open or closed
open rectangle gable vents are the functional choice when this opening is part of the attic plan. closed rectangle gable vents give you the same frame and slat shadow with no air path. Closed rectangles are useful on a gable that is already vented at the ridge or the soffit and only needs the look of a vent.
A short sizing note
LORT1212, a 12 by 12 inch rectangle gable vent is a square, and it is one size in a larger family of widths and heights. Do not assume every rectangle is 12 by 12. Pick the overall width against the gable using the appearance guide on the sizing page, then read the height on that specific product. Net free area, if you need it for ventilation, is listed on the product page and is not the same as the overall size.
Pair it with
Pair a rectangle with flat casing, a simple header, or shutters that are already rectangular. If the gable has an arch window, an arch top or a half-round will agree with it better. The install overview explains the opening for an open vent and the surface mount for a closed one.