There are some appropriate times to double barrel a name. It seems the origin is lost these days.
Double-barrelled names may be formed for a variety of reasons. Some are formed when the family names of two people are combined upon marriage or civil partnership, thus forming a new two-part surname, or when children are given surnames combining those of both parents. Double-barrelled names may also be used by children who are not brought up by their birth-parents to combine the surname of a birth-parent with that of an adoptive parent, or the surname of their biological father with that of a stepfather. Other families believe that the act of consistently passing on only the father's name is patriarchal in nature and choose double-barrelled names for feminist reasons. Similarly, Same-sex couples may want to emphasise equality between partners. In some cases, children whose parents divorce have both surnames, by parents' choice or their own choice if they are of the required age. To avoid the use of double-barreled surnames, a small number of couples create a new blended surname, combining parts of each name into one, such as Villaraigosa.
Among nobility, in the past especially, if a woman married down from her social status it was common for her and her husband to use a double-barrel name. This was done both so as not to diminish the social status of the woman and to gain for her husband a higher social status.
Double-barrelled names are sometimes adopted when the man has a common surname such as Smith or Jones which the couple want to avoid after marriage; hence double-barrelled names often incorporate a common surname. For instance, if Mary Howard married John Smith, they could choose to become Mary and John Howard-Smith (with the man's surname usually going second). In the vast majority of cases, though, they would simply opt for Mary and John Smith.
Some double-barrelled names are formed in order to prevent a family name otherwise dying out, due to the lack of males in a generation or when notable property is inherited through the female line with a stipulation that the individual inheriting use the family name.
For same-sex couples and their families, the presumptions of gender are irrelevant; many agree on one name or another (usually the more aesthetically pleasing), or combine the two as a double-barrelled name.
In Spanish- and Portuguese-speaking countries, having a non-hyphenated double-barrelled surname is the norm, and in some cases (e.g., Spain) it is actually a legal requirement. See the article on Spanish naming customs for more information.
