Here's what I did on this page to find possible relatives.
http://blancas.paginasamarillas.es/jsp/home.jsp
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Start by going to:
http://blancas.paginasamarillas.es/jsp/home.jsp
First, you may have to click on "cerrar" to close the ad. It may reappear every time you go to the blank form.
Then choose: Particulares y empresas, and not Organismos públicos.
Particulares means "individuals".
The instructions say, "Rellena, al menos, 1 de los 3 campos:"
Which means, "Fill in at least one of these three fields."
Nombre / Razón social
These mean "first name" (for nombre) and "business or organization name" (for razón social).
Leave them blank, unless you know the first name of the person you're looking for.
Primer apellido
This means "First last name".
The people this search finds will be children of a male with your forebear's last name (or the specific person who you already know with this first last name).
Start by adding one of your family member's last names here. If your family member was José Canga Velasco, his first apellido is Canga, which is from his father's side.
You should do this search again with Velasco, his second apellido, to find other possible relatives from his mother's side.
Note that you should search for Canga under both first and second last names because you will find relatives under both options. (But only put the last name in one field at a time.)
Segundo apellido
This means "Second last name".
The people this search finds will be children of a female with your forebear's last name (or the person who you know with this second last name).
So, do another two searches by adding one of your family member's last names here. If your family member was José Canga Velasco, his second apellido is Velasco.
Remember to use both first and second last names (Canga and Velasco) in this field to find possible relatives. (Leave the first apellido field blank, unless you are looking for someone you already know).
Provincia
This means "state or province".
Here put asturias.
Localidad
It basically means "county".
Here put langreo.
Calle
This means "street or road".
Leave this blank, unless you know the street they lived on and think the family may still live there.
Nº
Leave this blank, unless you know the house number of someone who may live in a specific home.
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Sandy, I found over 60 people. I've sent you three names, addresses and phone numbers of likely candidates.
I hope you speak Spanish, because you now have to either write some letters or make some phone calls. If you don't speak Spanish, perhaps there is someone near you who could sit with you while you do this.