subframe
noun
sub·frame
| \ ˈsəb-ˌfrām
\
\
variants:
or sub-frame
plural subframes
or sub-frames
Definition of subframe
: a secondary frame
especially
: one that is used for the attachment and support of a finish frame (as of a door or window) or of finish panels (as of a wall)
Strictly speaking, the vertical members of a curtain wall's subframe are called mullions, while horizontal members are called transoms. — Jason Alread et al. The rolled steel sections, generally 1/8" thick and 1" - 1 1/2" wide, were used for all the components of the windows: sash, frame, and subframe … — The Preservation of Historic Architecture: The U.S. Government's Official Guidelines for Preserving Historic Homes