Tuning, temperament and Pitch.

I've always been fascinated at the level of international cooperation between pipe organ builders over the centuries. Throughout Europe (France, Germany, Netherlands, Italy and Spain) the standardization of pipe length always seems to have been about the same. In the 1600's the pipe organ in Europe had reached a high level of sophistication. Although the British organ in the 1600's was woefully inadequate compared to those from France, Holland and Germany at that time, it seems that they set the standard for pipe length which has remained to this day.

Even when the metric system was created by France in 1795, and universally adopted throughout Europe, many pipe organ builders still continued still to use the British unit of measure ( the foot) to determine pipe length. There are some exceptions, like the 1863 Cavaille-Coll organ in the Santa Maria Basilica, San Sebastian, Spain. There are, of course, many organs with stops of no length designation at all.

In England, I came across some really old pipe organs, where the stop length had not been designated at all. The Standard pitch for the manual was 8 foot and that of the pedal, 16 foot. The Pedal Bourdon was an octave lower than the manual so sounded like a 16' stop.
The Manual Open Diapason I assumed was at 8 foot. The Double Open Diapason was assumed to be of 16 ' pitch. The Principal was usually considered a 4' stop, the Twelfth was a 2-2/3 and the Fifteenth was considered 2' pitch. A Mixture could be anything. A Trumpet was assumed to be the same pitch as the Open Diapason ( i.e.8 ft) and the Clarion the octave of the Trumpet. A pedal Posaune, Trombone or Bombarde was assumed to be of 16' pitch and a Contre-Bombarde of 32' pitch. There are also descriptive terms that leave nothing to the imagination like Double Open Wood and Untersatz. I'm assuming that most Double Open Wood stops are of 32'pitch.

Although organ builders adopted a stop deignation unique to themselves I always thought it was interesting that even after the adoption of the Metric System, the foot deigantion remained.

Some organ builders continue to use archeic designations from the 1300's that we are all familiar with today like Twelfth and Fifteenth that never actually specified the pipe length but just it's relationship to the standard pitch (8 foot).

Today, despite the Metric System being the Universal Unit of measure , pipe organ builders still use the foot as the standard unit of measure.

I read that before the foot system, pipe organ builders made the longest or biggest pipe, then made the rest of the organ to match. The problem was that you'd get wildly different pitch centers from organ to organ. The foot isn't actually unique to Britain, it was used prior even from Minoan times as a standard of measurement for trade, same with the pound, the inch, the talent (as much as fits in a standard amphora), etc. The metric system is a relatively recent invention, in the history of the world. Prior to that, every nation used some kind of foot (though they all had different names for it, the unit of measure was aways of very similar dimensions).

Remember, though, that the units (foot, inch) weren't standardized until late in the 19th century. They varied even between cities. There's a handy chart from 18th and 19th-century sources on the relevant wiki page: https://en.wikipedia.org/wiki/Foot_(unit)

Musical pitch also varied between cities. Wind instruments had to be constructed specifically for each place.