This spool holder is adjustable for different thicknesses of spools.  This feature keeps the spool from wobbling on the holder.  Just take out the plate and slide it to the appropriate slot in the spool holder.  The overall width should be thin enough to fit inbetween the walls of most filament spools that are already on the shelf (seen in pictures).  If the placement of the spool requires the spool holder to be placed in between to spools on the shelf, there is a slot in the middle to allow for such placement (seen in pictures).