In the Western tradition of surnames, there are several types of double surname (or double-barrelled surname). If the two names are joined with a hyphen, it may also be called a hyphenated surname. The word "barrel" probably refers to the barrel of a shotgun, as in "double-barreled shotgun".
In British tradition, a double surname is heritable, and mostly taken in order to preserve a family name which would have become extinct due to the absence of male descendants bearing the name, connected to the inheritance of a family estate. Examples include Harding-Rolls and Stopford Sackville.
In Hispanic and Portuguese tradition, double surnames are the norm, and not an indication of social status. A person will take the (first) surname of their father, followed by the (first) surname of their mother (i.e. their maternal grandfather's surname). The double surname itself is not heritable. These names are combined without hyphen (but optionally combined using y "and"). In addition to this, there are heritable double surnames (apellidos compuestos) which are mostly but not always combined with a hyphen.
In German tradition, double surnames can be taken upon marriage, written with or without hyphen, combining the husband's surname with the wife's (more recently the sequence has become optional under some legislations). These double surnames are "alliance names" (Allianznamen) and as such not heritable.
Many double-barrelled names are written without a hyphen, which can cause confusion as to whether the surname is double-barrelled or not. Notable persons with unhyphenated double-barrelled names include prime minister David Lloyd George, the composers Ralph Vaughan Williams and Andrew Lloyd Webber, military historian Basil Liddell Hart, evolutionary biologist John Maynard Smith, astronomer Robert Hanbury Brown, actresses Kristin Scott Thomas and Helena Bonham Carter (although she has said the hyphen is optional), comedian Sacha Baron Cohen (however, his cousin, the clinical psychologist Simon Baron-Cohen, opted for the hyphen) and Sylvia Llewelyn Davies.
A few British noble or gentry families have "triple-barrelled" surnames (e.g. Anstruther-Gough-Calthorpe; Cave-Browne-Cave; Elliot-Murray-Kynynmound; Heathcote-Drummond-Willoughby; Smith-Dorrien-Smith; Vane-Tempest-Stewart; George Henry Lane-Fox Pitt-Rivers). These indicate prima facie the inheritance of multiple estates and thus the consolidation of great wealth. These are sometimes created when the legator has a double-barrelled name and the legatee has a single surname, or vice versa. Nowadays, such names are almost always abbreviated in everyday usage to a single or double-barrelled version. For example, actress Isabella Anstruther-Gough-Calthorpe calls herself Isabella Calthorpe.
There are even a few "quadruple-barrelled" surnames (e.g. Hepburn-Stuart-Forbes-Trefusis, Hovell-Thurlow-Cumming-Bruce, Montagu-Stuart-Wortley-Mackenzie, Plunkett-Ernle-Erle-Drax and Stirling-Home-Drummond-Moray). The surname of the extinct family of the Dukes of Buckingham and Chandos was the quintuple-barrelled Temple-Nugent-Brydges-Chandos-Grenville.
Captain Leone Sextus Denys Oswolf Fraudatifilius Tollemache-Tollemache de Orellana Plantagenet Tollemache-Tollemache is sometimes quoted as the man with the most ever "barrels" in his surname (six), but in fact all but the last two of these (Tollemache-Tollemache) were forenames.