In the past, bicycle mechanics had to be aware of standards. This is less common with modern bikes because most are made in Asia, and they all conform more-or-less to British Standard component dimensions.
The standards that were common in a previous era were British, French, Italian, and Schwinn.
A very uncommon standard was Whitworth, also known as “British Standard Five,” “BSW” and “BS5,” found only on old bikes made in the British Empire. Whitworth is based on fifths of an inch. Not only are the threadings non-interchanheable, most wrench sizes are different as well.
The old-time mechanics, and people today working with vintage bikes, have to be careful about threading. For instance, you can screw a French freewheel onto an English hub, and although a bit stiff going on, all will seem well, until you ride the bike up a hill. The threads will strip, irrevocably damaging the hub.
Whereas the English standard used inch-based sizing, the French mostly used metric-based. To confuse matters, some French components used inch sizes. For instance, many French bikes were made with 27-inch tires.
Italians also used mostly metric but some inch sizing as well. The Italian threading, as well as Whitworth, used a 55-degree thread pitch angle, which can be problematic when things appear to match, but result in weak engagement.
Schwinn used to be in a world of its own. Whereas many of the English standard threaded parts used 24 threads per inch, Schwinn preferred 26 threads per inch. This caused no end of troubles for home mechanics as they would try to screw Schwinn axle nuts onto Huffy or Murray axles, resulting in badly damaged threading. The common theory is that Schwinn wanted to lock up the market for “Schwinn Approved” replacement parts. Schwinn also used a different tire sizing, sometimes interchangeable with EA1. Most tires were made to the EA3 standard, which although nominally the same, didn’t interchange at all. For instance, a typical tire size of 26 x 1.75 would not fit a Schwinn 26 x 1-3/4 wheel.
See also: Metric and Inch.