Have you ever looked at a big building and noticed that the windows seem to be in a big grid pattern? That's because the windows are held together with something called a glass mullion system!
The mullions are the parts that go in between the windows, almost like big picture frames. They are usually made out of metal or sometimes wood, and they help hold the windows securely in place.
But with a glass mullion system, the mullions themselves are made out of glass! This means that instead of seeing big metal or wooden frames when you look at the building, you see a smooth, seamless pattern of glass.
This can make the building look really cool and modern, and it also helps let more natural light into the building. Plus, because the mullions are made out of glass, they don't rust or corrode like metal mullions might.
So next time you see a big building with lots of windows, see if you can spot the glass mullion system!