The use of testable backflow preventers has been documented for more than 100 years. The Journal of the New England Water Works Association reported the evolution of the double check valve in the early 1900s. At the turn of the last century, the Journal stated that eight check valves were installed in series between the city and auxiliary water supply, yet backflow continued. The check-leakage problem was resolved when it was determined that metal-to-metal seating of checks would not seal due to corrosion. The solution was to have either the check or seal designed using a soft material, like leather.