These four (4) sheets of adheisve backed (PSA) Fish Paper thicker than the regular standard sheet. Each sheet  is 9″ wide x 10.5″ long and 20 mil (20/1000″) thick. They are equivalent to one 18" x 21" adhesive backed sheet. Twice as thick as the standard paper. Used in special applications where two layers of the 10 mil paper was used in the past. Color is very dark as shown in the photos.