It is a subordinate clause.
Euclid wrote them
Yes, Euclid wrote Elements.
He wrote Euclid's Elements.
Euclid wrote the famous book called Elements
Euclid
Euclid's Elements is the name you're after.
Elements
Euclid
Euclid
I wrote an incomplete sentence.My essay was incomplete.
'Elements' was written by Euclid.