Mullion
/ˈmʌliən/
Definition 1 of 2
Pro Tip 1/3Architectural Feature
Used to describe parts of a building's windows, not just any vertical bar. 
The modern office building's facade was notable for its glass walls and slim mullions.
Pro Tip 2/3Visual Separation
Mullions visually break up large glass areas, making windows more interesting. 
The designer added mullions to create a pattern on the vast glass surface.
Pro Tip 3/3Supportive Role
Mullions can also provide structural support to windows. 
To prevent the large window from sagging, they reinforced it with a sturdy mullion.