The stone framework that divides and supports sections of a Gothic stained-glass window is called tracery.
Tracery developed as Gothic builders enlarged window openings and needed a strong, elegant way to organize them. It consists of shaped stone members that divide the opening into narrow lights and curving ornamental patterns. The glass panels are fitted within these compartments, while the masonry carries structural loads around the opening.
Early Gothic windows often use simple bar tracery, in which stone bars rise from the supporting wall. Later Decorated Gothic designs feature flowing or geometric patterns, including circles, cusps, and elaborate branching forms. The tracery therefore contributes both engineering and visual rhythm.
A mullion is an individual vertical divider, whereas tracery is the broader framework or pattern. A flying buttress supports a building externally, and an archivolt is a band of molding around an arch.