You can define mullions by specifying a width and a depth. 

 Specifying mullion width and depth 
 
Tip:  To remove mullions for butt glazing, create a definition with both width and depth set to zero. Then, assign that definition to the mullions that you want to remove. 
-   Click 

. 
  
-  Expand Architectural Objects, and expand Curtain Wall Unit Styles. 
 
- Select a curtain wall unit style. 
 
- Click the Design Rules tab. 
 
-  In the left pane, select Mullions under Element Definitions. 
 
- Click New Mullion. 
 
- Enter a descriptive name for the mullion. 
 
- Specify a width and depth for the mullion. 
 
-  Specify the offsets you want. 
 
- Click OK. 
 
 After you create a mullion definition, you can assign it to any mullion in a curtain wall unit.