Abstract: A window system has matching groups of window sashes which enable the creation of a great number of different window array patterns using modular, interchangeable sashes. The sashes have light-dividing mullions which intersect with the sash frame at a particular level or width point such that another sash when positioned adjacent to the sash will have its mullion or mullions positioned to continue the same mullion line through the two sashes. In preferred embodiments the sashes are all rectangular, some groups with linear mullions and some groups with diagonal, arcuate or elliptical mullions.