How do I control curtain wall panel divisions and add mullions?

Select the curtain wall and open Edit Type: the Vertical and Horizontal Grid Layout settings (Fixed Distance, Fixed Number, or Max/Min Spacing) drive the panel divisions. For local tweaks, Tab-select an individual grid line to move it, unlock it, or remove a segment. Then add mullions — note that Interior and Border mullions are set separately per grid direction. If you need a custom mullion profile, duplicate one under Families → Profiles → Curtain Wall Mullion and adjust its dimensions. Full steps: Curtain wall grids mullions →