Mullion systems comprise a remarkably diverse range of novel structural solutions in facade applications. The simplest form includes a steel or aluminum section positioned at every vertical joint in the glazing grid. These steel and aluminum sections can be designed in open or closed positions. Mullion components often consist of built up custom structural sections capable of accommodating long spans in both vertical and horizontal positions (vertical being the most common). An interesting variation of this system type, however, employs horizontal mullions suspended from overhead cables and fixed to anchoring building structure at its ends. Mullion systems also include hierarchical structural frames and braced frames.

The vertical mullions at the L.A. Live Tower & Residences are custom built-up AESS components. Note the point-fixings that support the glass, and the mullion detail at head and foot that taper to a pin connection. A mullion is located at every vertical joint of the glazing grid.

Mullion systems can accommodate facade faceting with relative ease. Complex geometries such as the Spertus Institute, however, can produce many unique glass types.

The built-up horizontal mullions at the Orange County Performing Arts Center are hung from above with stainless steel rods and designed to fix the glass along the horizontal glazing grid.