Back in 1841, there was a need to standardise threaded fasteners and a chap called Joseph Whitworth devised the Whitworth system. It used a 55-degree thread form with rounded crests and roots. The across-the-points dimension of the hex head was twice the nominal thread diameter, making the across-the-flats dimension 1.732 times the nominal thread diameter. This didn't matter because everything was standard and a 1/2" Whitworth spanner (wrench) was one that would fit a 1/2" fastener. The Whitworth system was adopted as a British Standard and is often referred to as BSW.
Other standards were developed elsewhere, notably the USA, and these tended to use a 60-degree threadform and to measure the hex heads across the flats. Both systems are perfectly sensible, but it can be confusing when somebody used to one system has to deal with the other.
To confuse the Whitworth spanner/wrench issue further, there is a finer-threaded version of the threadform, BSF, which uses a head one size down from the coarser-threaded BSW. Whitworth threads are coarse, broadly equivalent to UNC and were initially developed with Wrought Iron as the main material from which fasteners were made. A decade or so after Whitworth's standard, the Bessemer process was patented and steel became widely used instead. The USA pretty much stopped using Whitworth and adopted the 60-degree threadform that was later to become UNC/UNF.
The Whitworth thread actually had a number of advantages over the UNF/UNC thread. The rounded root made Whitworth more fatigue-resistant (a rounded-root version of the UN thread was developed for highly-stressed applications and this became UNJ, adopted in 1965, 124 years after Whitworth). Anecdotally, Whitworth threads are much preferred by quarrymen, and folk in similar industries, because they are less prone to seizing (freezing?) and can usually be dismantled somewhat easier than other threads. The most likely reason seems to be that both the crests and roots are rounded, giving a closer fit and less of a path for water and contaminants to get in than the UN threads, which truncate the thread peaks.