Well, it depends on whether we are talking about first names or last names, both of which the nobility famously had a lot of. As far as last names are concerned, most of the times it had to do with estates, lands or titles that belonged to different families and were eventually inherited by the same person. Suppose you held an ancestral lordship over the town of Reddit: your last name could very well be "de Reddit"/"von Reddit". Now, two situations could arise which would make that name change: 1-You inherit another lordship, over the town of Spotify, from your mother's family (the "de Spotify"), and thus start carrying the surname "de Spotify de Reddit"/"von Spotify-Reddit"; 2-You are a younger son (or descended from one) of the old "de Reddit" family. However, all you have is the surname, since the lordship is held by your cousin. Eventually, you are either granted or inherit another lordship over another town (Youtube). You adopt your new lordship as your last name, but you don't give up your ancestral name (because it carries a lot of prestige). Hence: "de Reddit-Youtube"/"von Reddit-Youtube". Examples of this would be the: 1)Saxe-Coburg-Gotha-Branganza 2)de Bragança Sousa e Ligne de Portugal e Castro Álvares Pereira de Melo In many cases, adopting the surname would be a pre-requisite for inheriting the estate, making many people adopt surnames to lordships they merely had a claim to, in order to reinforce that claim.