Even though it often seems that there are no rules for showing possession, there are. Chicago documentation style gives 11 general rules for showing possession. Our casual writing style today often ignores these conventions, but academic discourse requires accuracy. In other words, get it right!
To form a singular possessive: (one person "owning" something)
To form a plural possessive: (two or more people "owning" something) add just an apostrophe (') to the end of the plural noun.
Example:
The politicians' votes were all negative.
To indicate joint possession: Add an apostrophe s ('s) to the second noun if both "own" the noun.
Example:
Joseph was Rachel and Jacob's child.
To show separate ownership: Add an apostrophe s ('s) to each noun.
Example: Joseph was Rachel's second and Jacob's eleventh child.
There are about as many exceptions to the rules as
there are rules. Grammar is often frustrating so
if you are unsure look it up!