Whitworth

Sir Joseph Whitworth proposed this thread in 1841. This was the first standardised thread form. The form of the thread is shown in the diagram. The principal features of the British Standard Whitworth (BSW) thread form are that the angle between the thread flanks is 55 degrees and the thread has radii at both the roots and the crests of the thread.  (emphasis mine)

Most common machine screws use an angle of 60 degrees in the 'V' between threads. Whitworth, in his most benevolent wisdom, used 55 degrees. I am sure he had a very good reason for it. Not. He was British. He probably drank tea.

Jack found the ultimate definition of "arcane": a web page devoted to obsolete British machine screw thread standards. Back when I had my Triumph I needed three sets of wrenches to work on it:

1. American for the engine.
2. Metric for the front brake.
3. Whitworth for the chassis.

At that point I had never heard of Whitworth. Fortunately the former owner had a small set of Whitworth wrenches that would suffice for any chassis work I needed to do. He handed them over along with the motorcycle, and why not? What was he going to do with them? Go look for another English motorcycle that used Whitworth fasteners? No-thank-you-very-much.