4 hours ago, drpaladin said:

Most homes with sash windows are simple grid windows rather than millions. Mullions were a practical element at the time because they added structural support and larger pieces of glass were expensive.

yes indeedy

especially in the UK, mullioned traditionally meant structural divides between the windows, made of stone (sometimes brick or wood) in older buildings, comprising small glass panes (basically that’s all you could get then) held together by lead strips - called “leaded lights” - which might be fixed or side opening (casement), and then later (Georgian) windows used slender wooden glazing bars (vertical + horizontal) to hold together larger panes then available in a fixed or sash (sliding) window. Now, with new materials, the terms get interchanged