Im a hydraulics engineer and you dont want to get into thoes threads!!!
In hydraulics for example, 1/4" BSP thread to 1/4" hosetail (on a fitting) are completely different!!
Think about it, 1/4" is roughly 6.5mm (tail size) but the threads are about 10mm (guessing as no guage with me in office!)
JIC used to get me at first then it all fell into place! Some people use thread sizes but also can use DASH sizes
1/4" is -4
5/16" is -5
3/8" is -6
1/2" is -8
5/8" is -10
3/4" is -12 etc
With JIC its a case of
7/16" is -7
1/2" is -8
9/16" is -9
5/8" is -10
3/4" is -12
7/8" is -14
1 1/16" is -17
1 3/16" is -19 etc
