Simplify the following text for a Grade 9 native English-speaking person.
Original: This perpendicular linearity is particularly obvious in the design of windows , which became very large , sometimes of immense size , with slimmer stone mullions than in earlier periods , allowing greater scope for stained glass craftsmen .
Simplified: This is particularly obvious in the design of windows , which became very large , sometimes of immense size , with slimmer stone mullions than in earlier periods . This allowed more scope for stained glass craftsmen .