The Probert Encyclopaedia of Architecture

TRACERY

In architecture, tracery is the stone framework in the head of Gothic windows, formed by a continuation of the mullions, bent, as it were, into ornamental designs. It was at first confined to circles and other geometric forms; but later the lines were free and more flowing except in the Perpendicular style, in which the mullions were carried right through in straight lines.
Research Tracery