What is the name of the horizontal bar that intersects a window mullion?

Mullioned windows are windows divided into panes by inner frames. The vertical frames are called mullions. What do we call the horizontal frames?

Solution 1:

Transom according to Wikipedia, which says "In architecture, a transom is a transverse horizontal structural beam or bar, or a crosspiece separating a door from a window above it. This contrasts with a mullion, a vertical structural member." Sadly I can't find a more authoritative source although various glazing/window/building companies use the term, e.g. Architonic, Thames Valley windows.

• https://en.wikipedia.org/wiki/Mullion
• https://en.wikipedia.org/wiki/Transom_(architecture)

Solution 2:

In the usage I'm familiar with (doing some home restoration in the western US), the members dividing individual panes of glass (whether vertical or horizontal) are muntins.

I hadn't run into the term mullion before, but apparently it is used for vertical members that separate sashes or casements.

In paneled (rather than glazed) doors, the horizontal members dividing the panels are rails and the vertical members are stiles. The larger horizontal members of a sash may also be called rails.

Image source: Twitter user @tylor

Image source: City of Burlington (WI) Historic District