Select M12 in Size, and click OK.
A nut appears in the assembly.
Drop a second nut on the edge of the second hole.
Click
.
The nut sizes automatically to the most recently used size.

Because of
SmartMates and Mate References on the SolidWorks Toolbox components, the
software automatically adds mates between the block and the nuts. A Concentric
mate aligns the nut to the hole and a Coincident mate aligns a flat face of the nut to the
block.