The United States and English double-protection masks consisted of 11 principal parts as follows:
1. A knapsack slung from the shoulder or neck. This contained the canister and a pocket for storing away the mask when not in use.
2. A metal canister in which was contained the absorptive neutralizing chemicals.
3. A flexible hose reaching from the canister to the face piece.
4. A flutter, or exhalation, valve, which opened when the wearer exhaled his breath and closed when he inhaled, thus bringing the inhalation through the canister but allowing the exhalation from the lungs to pass out without polluting the chemicals of the canister.
5. The face piece, or hood, fitting snugly around the edges and covering the eyes, cheeks, lower forehead, nose, mouth, and chin.
6. The eyepieces, or lenses, through which vision was maintained.
7. An elastic harness for the head, to hold the face piece in place.
8. A body cord to tie around the chest and hold the knapsack firmly, so that the mask could be seized in both hands and pulled out of the knapsack.
9. A metal flange connection or angle tube which carried the hose through the face piece to the mouthpiece.