surnames

I AM CONFUSED AS TO THE USE OF SURNAMES. I HAVE SOMEONE NAMED GERTRUDIS GONZALES DE PLACENCIA. WHICH NAME IS HER FATHERS AND WHICH NAME IS HER MOTHERS? WHAT DOES "DE" SIGNIFY? AFTER SHE MARRIES IS THEIR A CHANGE? WHEN LISTED IN AN ALPHABETICAL INDEX HOW IS SHE LISTED AS? DOES SHE GO UNDER G FOR GONZALES OR DOES SHE GO UNDER "DE" OR IS SHE UNDER P FOR PLACENCIA? WHAT IF SHE REMARRIES? THANK YOU.

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