CONCEPT
Reverse Salient
The lagging component in a sociotechnical system that constrains overall advance—where innovation effort concentrates and where the system's trajectory is actually determined.
Hughes borrowed the term from military history, where it denotes a section of an advancing front that has fallen behind, creating vulnerability. In large technical systems, a reverse salient is the component whose lag limits the system's overall performance and attracts concentrated innovative effort. The direction of a system's evolution is determined not by its most advanced component but by its most retarded one. When Edison's electrical system had functional generators and distribution networks but lamps that burned out too quickly, the lamp was the reverse salient—Edison's famous filament search was effort to resolve the bottleneck. Once resolved, the reverse salient migrated: the limited range of DC distribution became the new constraint. Systems advance through successive resolution of reverse salients, each resolution revealing the next bottleneck.
In The You On AI Field Guide
The concept is diagnostic rather than prescriptive. It identifies where the system's development is actually constrained, directing attention away from components advancing rapidly (which attract the most public attention and investment) toward components whose lag determines the system's trajectory.