Mr. Tamarkin says he uses pricey steel casement windows as opposed to aluminum because the mullions are slim (“I don’t like fat-mullioned windows,” he says) and because the metal shows ...

The mullions were cut on-site and fitted with ... while a diagonal-set casement window opens like a door with a right-hand hinge. “We used the hopper close to the tub in case a child is bathing ...

In this project by Yiangou Architects, a bank of bronze casement windows, with mock leaded lights fixed with a traditional bead, are set within hand-carved stone mullions. Cassetta Finestra terracotta ...

When used in this way, purely as a decorative element, muntins can also often be referred to as "mullions" or "grilles ...