Is my surname Rodríguez or Martín? Should citations say [Ro] or [Ma]?
Short answer: My first surname is Rodríguez, so I would cite my papers as [Ro], not [Ma].
Long answer: People in Spain usually have a given name (nombre) and two surnames (apellidos).
First surname:
the first surname of a person X is typically the first surname of X’s father.
This is the main surname and the one normally used in everyday contexts.
Second surname:
the second surname of X is typically the first surname of X’s mother.
It appears mainly in formal situations where the full legal name is required,
in a role similar to a middle name in English.
So I would write my name as:
Saúl Rodríguez (everyday use)
Saúl Rodríguez Martín (more formal, including the second surname)