Double-edge (DE) safety razors comprise the bulk of safety razors made and sold. The idea of a double-edged blade was patented by King Camp Gillette around 1901 and sold starting in 1904. Numerous single-edge razors were in production then, so Gillette's advertising included a description of how many "shaving edges" per box, rather than razor blades per box. Gillette's move to sell razors to the US military, as it was mobilizing troops for World War I, cemented Gillette's place as the US sales leader in shaving, and thus DE blades occupied a place as at least the first among equals, if not the outright dominant shaving system, on the market for perhaps 60 or more years. Gillette created the "loss leader" model (often referred to as the "razor and blades" model) of production and sales: the manufacturer sells the razor itself for minimal or even no profit, only to reap profits on razor blades for many years. Gillette's use of patents has provided them with at least temporary monopoly position as provider of razor blades/cartridges as they introduce new models of razors. That is, until the patents expire, other manufacturers are discouraged from entering the market producing Gillette-compatible razor cartridges. Fortunately for DE shavers, the initial patent on DE blades expired long ago, and there are a number of blade marques from which to choose.
DE blades are thin enough to flex when bent, whereas SE blades generally are thicker and flex much less, if at all. As a result, DE razors most often bend the blade into a slight curve, and SE razors are "hoe-type", with a flat surface holding the blade.