The Metric System
Having worked in the aviation maintenance field most of my adult life I've seen it evolve...
The US military manuals were published in US inches almost exclusively until around the early 1970s.
New development since has been for the mos t part void of any size references in procedures. Dimensions shown in charts/diagrams are required to show dual dimensions (US Standard & Metriv)
As a Technical writer for an international parts manufacturer, and writing to (ATA - Airline Transportation Asscociation) Standards, EVERYTHING is displayed in dual format.
Metric makes sense to me, with one exception -Temperature is silly in °C!