The difference between a flush mount vent and a drop in vent is how they are installed. When installing a flush mount vent, the flooring must be notched and cut to precisely fit the outside piece of the flush mount vent. Once that has been installed, the center piece can be dropped in and removed for cleaning or accessing the duct. Some people prefer the flush mount vents over drop in vents because once installed, the vents are completely flush with the flooring. We also have this vent in a slightly larger size here. Please look at the diagrams on both pages to see the size differences.