Double sided and waterproof due to a high density rubber seal. Extra depth on one side for really large patterns. This box is made from ABS (Acrylonitrile Butadiene Styrene) which is an opaque thermoplastic and amorphous polymer.
The Tackle Shop "Flybrary of Congress" measures 12 3/4" X 10" X 3 1/2
One of our favorite boxes for big flies and a lot of them