1843 E. Bulwer-Lytton I. ii. iii. 193 A large mullion window.
a1933 J. Galsworthy (1934) i. i. 3 He lay in a room with mullion windows.
2001 (Electronic ed.) 9 Mar. 8 Local stone was then used to encase the inner structure, there are lovely mullion windows and diagonally-set chimney stacks.