What characterized Armstrong's two-speed hub developed around 1904?

It typically featured two different-sized sprockets, one on each side of the hub shell.

The Armstrong two-speed hub, developed around 1904, represents an early, integrated approach to variable gearing centered on the rear wheel mechanism. Unlike later, more complex internal hubs, this design functioned on the principle of a 'flip-flop hub.' This meant the hub shell contained two distinct sprockets of different sizes—one affixed to each side. To change the gear ratio, the rider had to stop the wheel rotation, dismount if necessary, and physically flip the entire rear wheel around so that the chain engaged the sprocket on the opposite side. While this offered an advantage over a fixed gear for hills, the shifting process itself was cumbersome.