Front Vent Glass or Rear Vent Glass may be a side window in the front door located on each side of the Vehicle, or in the rear door behind the rear drop glass. Only some cars have them.
The term Vent Window/Glass originates from older auto glass designs that were set on hinges with a small latch to hold them closed. Most often found on the front door, it is a small roughly triangular glass in front of and separate from the main window that rotates inward (see image) for ventilation.
Many early closed cars had front and rear vent windows called "ventiplanes". It has hinges and a latch, thus it can be opened for additional ventilation. Most vehicles since the 1960s have removed this feature for cleaner styling, known as "ventless" windows. Some auto makers continued to offer vent windows for increased flow-through ventilation. Although the front venting windows "provide unmatched ventilation, air turbulence and leakage outweigh the benefits". As automobile air conditioning became more popular, front window vents disappeared by the 1980s.
They can also be non-movable and mounted in the door itself because that section of the rear side glass would not be able to slide down because of the cut-out in the rear doors required to clear the rear wheel housings. The fixed portion of the glass is separated from the main window that rolls down by a slim opaque vertical bar.
In some automobiles the fixed quarter glass may set in the corner or "C-pillar" of the vehicle. There are also designs that incorporate two quarter windows, one that is part of the door and the second mounted in the roof pillar. This arrangement may help to increase driver visibility. In this case, the quarter glass in the C-pillar would not be called an "opera window". Non-opening, fixed quarter windows are sometimes installed like windshields in that they are bonded to the body with urethane,