The Spanish system of using two surnames, from the father and the mother, appeared in quite recent dates, in the second half of the 19th century.

Of course, we are talking about official standard procedure emanating from the Civil Registries and the Civil Code, but that does not mean it did not exist prior to that date, rather it indicates a definitive codification of a common practice.

The usage of two surnames came from the nobility, and started seeping through to the bourgeoise, robe nobility, or urban patriciate, so to speak, and from there it trickled down to middle classes, later getting to it being a common feature.

In the oldest days, surnames were more like monikers, ways of telling apart one person from another, like distinguishing Juan from Villalón de Campos, from Juan from Medina, or Juan the blacksmith, who would normally be referred to as Juan de Villalón, Juan de Medina, and Juan Herrero.

Nobility, on the other hand, tended to make their noble lineage, further pointing out how they were noble on both sides, and that is how in the 16th century you start seeing noblemen using two surnames. Examples abound like Fernán Álvarez de Toledo y Pimentel, Luis Colón de Toledo, or Pedro Fernández de Castro y Andrade. Contrast with prior nobles like the marquess of Santillana (Íñigo López de Mendoza) or his sons (Pedro González de Mendoza, Lorenzo Suárez de Figuerosa, Juan Hurtsdo de Mendoza, Leonor de la Vega). Nobility surnames were quite anarchical, as they varied widely, emphasising the different lineages related to different fiefdoms.

In the 19th century, with population growth and common surnames becoming even more common, it was deemed necessary to crrate a system to better identify and distinguish people on an official document basis, so the method of having two surnames became the official way.

This official naming convention came with another way of breaking nobiliary anarchy in nomenclature: all brothers would have to bear the same surnames in the same order. Thus, all brothers would carry the same surnames, making it easier for official matters to identify related people, which makes sense for the matters of inheritances ab intestato and ways of getting official notices to the appropriate people.

There are some execeptions to this rule, but they come from particular privileges granted to specific people. Think of the late duchess of Alba, Cayetana FitzJames Stuart y de Silva, married to Cayetano Martínez de Irujo y Artazcoz. She was a very important person, daughter of another very important person. Ordinarily, her children would be named X Martínez de Irujo y FitzJames Stuart, but some of her children carry the surnames in the other order, considering how mighty special the surname FitzJames Stuart is.

Occasionally, a judge may approve an inversion of the surnames in later life taking into consideration the rarity of a given surname, and hence the need for its preservation.