Muntin

Muntins are like little bars that separate a big window into smaller squares. Imagine having a big cookie, but then dividing it into smaller pieces using a knife. That's what muntins do for windows! They make a big window look like lots of little windows. This has been a popular design for windows for hundreds of years, and it's still used today. So, instead of having one big piece of glass in a window, we have lots of smaller pieces with muntins in between.