The origins of the double iron are somewhat mysterious, but probably date to the mid-18th century. A double-iron smoothing plane discovered in London during an archaeological dig may have been made as early as 1750. The most interesting feature of this plane is that while the business end of the cap iron is typical, there is no mechanism for fastening the cap iron to the cutting iron. The two irons are loose, held together by the wedge, so the owner would have to set both irons by tapping them independently. This arrangement supports the theory that the earliest double iron was simply two cutting irons that some enterprising craftsman placed back-to-back.
The first written reference to the double iron dates to 1767. In an advertisement in a Pennsylvania newspaper, Samuel Carruthers offered planes for sale, including “double-iron’d planes, of a late construction, far exceeding any tooth planes or uprights whatsoever, for cross grained or curled stuff.” Translation: double-iron planes, recently invented, perform far better than toothing planes or high-angle planes in wood with difficult grain.
–Steve Voigt, excerpt from “Cutting-Edge Technology: Rediscovering the Double-iron Plane” in Issue Six