Use this procedure to define mullions by specifying a width and a depth.
Because door/window assemblies can contain multiple nested grids with different mullions, it is helpful to use a naming convention for grid mullions that indicates the grid location or purpose within the door/window assembly. For example, L3-FL1- Window Mullion identifies a mullion for a window in a third-level grid on Floor 1.

Specifying Door/Window Assembly mullion width and depth


 Door/Window Assembly Styles. 
  
  
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After you create a mullion definition, you can assign it to any mullion in a door/window assembly.