The world's most common thread? This must be the 1/4" x 20 t.p.i. Whitworth form found in the base of a camera that allows it to be fitted to a tripod or other mount. It's common to every camera maker and every model they produce - though some heavier professional types have, as a concession, fitted with a more robust 3/8" x 16 t.p.i.
From this page: http://www.lathes.co.uk/latheparts/page6.html
The minor diameter difference is because Whitworth has a steeper thread, thus the root diameter is larger because the depth of the thread is shallower off of the major diameter of the screw.
A four year old thread revived!
Yes, One has a 55 degree cut angle and the other is 60 degrees. I forget which way round it is. As you say, if you put the screw of one into the thread of another there will not be a flat to flat surface bearing the force. It will work but will wear with continued use.
The original difference of opinion here was with which thread was used. I claimed Whitworth but others (usually Americans) thought it was UNC. The fact that many cameras were made with 1/4" x 20 tpi threads before UNC was unified makes me think that it was originally Whitworth.
The 1952 ASA standard tripod screw specification is 1/4 - 20 TPI UNC-1A thread (and 3/8 - 16 UNC-1A for larger cameras) on American cameras. Perhaps in England the Whitworth form was used. Several variations of T-nuts and threaded inserts in these sizes are readily available over the counter in Ace Hardware Stores in America. David Goldfarb gives good advice above on mounting a T-nut so the tripod pulls it against the wood of the camera rather than relying on little screws to hold the T-nut as seen in some imported pinhole cameras.
Ah, I suppose camera/head vintage now becomes important. I was basing some of mine off of the measurements I took from my cameras and fittings. It would make sense that a really old camera would have a different threadform.
My original understanding was that the discussion was counting diameter and pitch as the important pieces of a screw, not the form; rather than the idea that diameter/pitch has been relatively standard but threadform has changed.
As long as you don't do it up too tight, you can get away with a UNC screw in a Whitworth thread but eventually it will wear more than the proper mating threads.
When I looked into this a while ago I found one tripod manufacturer who specified UNC and another using Whitworth so there doesn't appear to be a standard.
Likewise, there is probably not a standard with cameras but I suspect that German and Japanese manufacturers are using Whitworth as that's what they were using before UNC existed.