Dear all,
Just to add my 2 cents to the Portuguese Names from Portugal...
Nowadays this is the rule (since 1995):
2 First Names + 4 Surnames max. (The surnames might be a perfect mix from both parents, just from one of them or even from ancestors, they are entitled to).
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https://irn.justica.gov.pt/Servicos/Cidadao/Nascimento/Composicao-do-nome
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http://bdjur.almedina.net/item.php?field=item_id&value=393046
(this second link has a bottom page with a reference to 1931 Names law)
However, the rule that most names nowadays abide by is still:
The NAME (max 2 names) + Mother SURNAME (max 2 Surnames) + Father NAME (max 2 Surnames) was settled by law in 1931.
By marriage, a wife could add up to 2 surnames from the husband. Also, since 1977 a husband could also adopt up to 2 surnames from his wife by marriage.
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https://www.publico.pt/2011/08/15/jornal/as-regras-dos-nomes-22698404
However, nowadays many women do not take by marriage the husband's name.
Before 1931 the rule was a bit of common sense as explained. However with the Civil Parishes starting around 1911 (after Portugal became a Republic), children were registered with the father's name in the end.
As for Catholic Parishes before 1911, you can only double check what the real name was at the time of marriage, widowing, christenings, last wills, or other events where the full or partial name was needed. So better to assume the patronymic, as a common rule.
Also, do not forget that some surnames might also come from a place or a craft, from Godparents (especially when they are abandoned children)
Examples:
1- In my family in the 1700s the husband "da Silva" took the wife Surname "Almada" forming a new family "da Silva Almada" (my branch lived until my Grandfather)
2 - Recently a cousin named his children with the combination of the following surnames:
1st Surname (Mothers' Mother Surname) 2nd Surname (Fathers' Mother Surname) 3rd Surname (Mothers' Father Surname) 4th Surname (Fathers' Father Surname)
3 - A mother's surname could/can become the family surname when there was/is no known father.