Four different sizes of plain floors to give you coverage and technical building options. Use the 1"x1"s for for filling little gaps, or as stepping stones for negative space. Use the 1"x2"s to center special floors when working with even dimensions. Suspend the 4"x4"s between any two elevation pieces to make narrow walkways. 

Combines well with any Core Sets for more coverage. Also helps your walls-to-floors ratio if you get multiple Walls and Doors add-on sets.