While not all automobiles are equipped with the exact same features, there’s a good chance the rear windows of your four-door passenger car don’t roll all the way down.
The seeming ubiquity of this trend is one reason you may have assumed it was some sort of safety-related mandate, as is the case with seat belts and airbags. Though backseat passengers aren’t always little kids, little kids are always backseat passengers—so it makes sense that the windows would stop short of allowing them room to wriggle right out of the car. The feature complements car doors’ child safety locks (which prevent anyone from opening the doors from the inside) pretty nicely.
But this safety element is actually just a convenient side effect of rear windows’ limited mobility. The real reason, as Gear Patrol explains, is because “there’s no place for the glass to go when the window rolls down.” If you look at a regular four-door car from the side, you’ll notice that the front doors are rectangular—giving the front windows a flat expanse to slide right into. The back doors, meanwhile, are usually curved or slanted forward to make room for the back wheels. This shape interferes with the path of the back windows: The glass sheets can only slide down so far before they’d hit the wheel wells.
Some designers have avoided this issue by simply splitting the rear windows into two panes. The larger one toward the front rolls all the way down, and the smaller one behind it doesn’t move at all. If you happen to be using one of those vehicles to cart around a couple kids, it’s probably best to be extra diligent about activating the child safety locks.
[h/t Gear Patrol]