I'm not English hence I don't know how this is called in the first place (that's why the title of this question should be changed) but what I'm referring to is this "s" used in English language to denote something belonging to someone/something or being related to something/someone:
I took my father's coat.
I wonder what that 's stands for? Does it abbreviate some longer word as it does when it abbreviates is or was (ie. He's tall)?
Why do we only use apostrophe when doing the same thing in plural:
I made clients' incomes grow.
I know my examples may be bad/invalid but I would like to know some theory behind this as well as any exceptions to the rule if they existed... I know it's different when you refer to something of the first person. We don't say I's coat but rather My coat. This probably is an exception.