[toomanybeersies]

So where to 3/8" and 5/16" threads play into this? They aren't easily divided by 2, or multiplied by 2 to make a larger thread.

[ArkyBeagle]

3/8 is half between 1/4 and 1/2. 5/16 is half between 4/8 ( 1/2 ) and 3/8.

[Turing_Machine]

And going the other way, 2 * 3/8 = 3/4, and 2 * 5/16 = 5/8, both of which are also standard sizes.

[rm445]

5/16" is the world's most compatible thread. The numbers are close enough that, in the most common thread pitch, you can usually get away with using it interchangeably with M8.