While I'm looking for a completely different tree. I found the postings helpful.
Katherine Aguilar

It was very common and normal for children to take on their mother's surname instead of their father's.

The current naming system in the Spanish speaking world did not come about until the Bourbon reforms.

Because use of maternal and paternal surnames were both very common, the convention was a bit of a compromise combining the use of both surnames.

As Linda mentions, it was often the custom that the oldest daughter was named after the maternal grandmother, including her surname so she would have a surname different from either of her parents.

this is one of the mysteries of genealogy that keeps us shaking our heads and
wondering if we have the correct lineage sometimes.. It's especially interesting
when they use a grandmothers name who you have not identified yet and the
grandmother used her grandmothers surname.. and yet we stay with it until we
unravel the mysteries.. the ancestors must have a good chuckle at us sometimes..
Linda in B.C.

________________________________
From: "sanjeronimolosbarbosa@yahoo.com"
To: general@lists.nuestrosranchos.org
Sent: Tue, July 12, 2011 10:33:10 AM
Subject: [Nuestros Ranchos] Surprising surname changes with ancestors?

Hi, I would like to share a quite funny, or surprising, discovery I have just
made. On the marriage act of my third great-great grandfathers José Gerardo
Tapia and María Josefa Gímenez on July 15, 1815; there was quite a tornado of
changes in their surnames.

José Gerardo Tapia, a mulato from Aguacaliente, Cocula; was the legitimate son
of José Antonio Placido Rodríguez and María Mauricia Tapia. So, his real name
would have been José Gerardo Rodríguez Tapia.

But I am certain that he his my great-great grandfather Gregorio Tapia's
paternal grandfather because family testimonies state that Gregorio was very
much dark-skinned. So it might have been that José Gerardo changed his surname
to Tapia for reasons yet unknown.

Second surname change is of his bride and then after wife María Josefa Gímenez,
who was stated as the an Amerindian laborer woman of La Sauceda, the legitimate
daughter of José Leonisio Gutiérrez and María Magsima (or Maxima) Ruelas. So her
real name would have been María Josefa Gutiérrez Ruelas.

In the baptism of one of their sons Seberiano Tapia Villegas, Josefa's surname
is changed as Villegas Jímenez, as it is stated that Seberiano's maternal
grandparents were León Villegas and Macima (again, Maxima) Jímenez. So,
Seberiano Tapia Villegas (a lobo or zambo, according to the cuadro de castas)
son of Gerardo and Josefa, was if the surnames were actually authentic:
Seberiano Rodríguez Gutiérrez.    -- -- -- -- -- -- -- -- -- -- -- -- --
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yes .... these surname changes were very common - there were no hard set
rules with surnames until more recent times, especially with the non
Europeans ... just be thankful that you seem to be able to make the
connections - it isn't always obvious.

-Angelina-