It's called a double-genitive. The following explanation is from
. I've marked the gist in
Another eccentric usage is in expressions that seem to mark a genitive relationship twice:
A friend of my daughter’s is a lawyer.
The genitive relationship m
y daughter is embedded into the genitive relationship `daughter
’s friend'. The structure of ... ’s allows these two genitives to be expressed in one sentence.
When using this double-genitive structure the writer must be careful to place the apostrophe where meaning requires that it be placed. Note the difference in meaning when the apostrophe of the previous sentence is placed after the
s :
A friend of my daughter’s is a lawyer.
A lawyer is the friend of my only daughter or of one of my daughters.
A friend of my daughters’ is a lawyer.
A lawyer is the friend of several or all of my daughters.
It is perfectly correct to do without this double genitive when the genitive relationship is between nouns: A friend of my daughter is a lawyer.
A friend of my daughters is a lawyer.
But we cannot avoid it when the genitive relationship is between a noun and a pronoun:
A friend of mine is a lawyer
A poem of his was published recently.
All the best. :-D