Explenation about inch theads.

Please somebody explain me something about inch threads. for eg. you have thread like this:

Is the information about threads per inch given two times? One time in TPI information and the second time in the UNC mark as it is also telling us how many threads per inch we have? For me it is the same information once given in exact number and second time by putting information that should take us finally to metric ISO directive of how many threads per distance we have in this exact example?

The UNC just states what standard the thread is. Coarse, Fine etc. that does to an extent change the TPI depending on the standard but other variables matter as well. A very aggressive thread pitch will decrease the overall TPI and vice versa. So you really need to know all three bits of information to find a proper mating surface.

EDIT: But in the US a standard drywall screw fits just about every possible use case…

So what you mean is that not only the TPI information is included in UNC marking but also for eg. inner diameter angle of the thread yes?

Yes the UNC standard will specify the height and spacing of the threads. But that does not necessarily determine the thread per inch.

my favorite phrase is “the good thing about standards, is that there are so many to choose from”.

So to sum it up. There may be different TPI value within for eg. UNC standard that not determine TPI exacly for one diameter of the thread(let's take #4 for example). But there is particular case of the UNC where TPI of this particular thread size #4 match te ISO metric standard. That is correct?

Sure. Each application is different but industries adopt certain favorites. In video production everything is 1/4 20. And eventually all manufacturers fall in line. Helps keep costs down also when something is widely utilized.