In history and heraldry, a cadet branch consists of the male-line descendants of a monarch or patriarch's younger sons (cadets). In the ruling dynasties and noble families of much of Europe and Asia, the family's major assets – realm, titles, fiefs, property and income – have historically been passed from a father to his firstborn son in what is known as primogeniture: Younger sons – cadets – inherited less wealth and authority to pass to future generations of descendants.
In families and cultures in which this was not the custom or law, as in the feudal Holy Roman Empire, equal distribution of the family's holdings among male members was eventually apt to so fragment the inheritance as to render it too small to sustain the descendants at the socio-economic level of their forefather. Moreover, brothers and their descendants sometimes quarreled over their allocations, or even became estranged. While masculine primogeniture became a common way of keeping the family's wealth intact and reducing familial disputes, it did so at the expense of younger sons and their descendants. Both before and after adoption of inheritance by primogeniture, younger brothers sometimes vied with older brothers to be chosen their father's heir or, after the choice was made, sought to usurp the elder's birthright (cf. Jacob and Esau).
In the parts of Europe where primogeniture prevailed, cadet sons were generally entitled to receive an appanage in patrimony, always substantially smaller than the eldest son's inheritance. Often, especially outside of Germany, the younger branch remained subordinate to the elder line as vassals or subjects.
Often, however, one or more younger sons were encouraged to take clerical orders, thereby forfeiting all rights of inheritance. Or a junior male might be encouraged to pursue a career in the military as an officer (e.g. Prince Eugene of Savoy), or as a courtier or civil servant in the monarch's capital.
In such cases, primary responsibility for promoting the family's prestige, aggrandizement, and fortune fell upon the senior branch for future generations. A cadet, having less means, was not expected to produce a family. If a cadet chose to raise a family, its members were expected to maintain the family's social status by avoiding derogation, but could pursue endeavors that might be considered demeaning for the senior branch, such as emigration to another sovereign's realm, or engagement in commerce, or a profession such as law, academia, or civil service.
In some cases, cadet branches eventually inherited the crown of the senior line, e.g. the Bourbon Counts of Vendôme mounted the throne of France (after civil war) in 1593; the House of Savoy-Carignan succeeded to the kingdoms of Sardinia (1831) and Italy (1861); the Counts Palatine of Zweibrücken obtained the Palatine Electorate (1799) and the Kingdom of Bavaria (1806); and a deposed Duke of Nassau was restored to sovereignty in the Grand Duchy of Luxembourg (1890).
In other cases, a junior branch came to eclipse more senior lines in rank and power, e.g. the Kings of Prussia and German Emperors who were junior by primogeniture to the Counts and Princes of Hohenzollern, and the Electors and Kings of Saxony who were a younger branch of the House of Wettin than the Grand Dukes of Saxe-Weimar.
A still more junior branch of the Wettins, headed by the rulers of the small Duchy of Saxe-Coburg and Gotha, would, through diplomacy or marriage in the 19th and 20th centuries, obtain the royal crowns of, successively, Belgium, Portugal, Bulgaria and the Commonwealth realms. Also, marriage to cadet males of the Houses of Oldenburg (Holstein-Gottorp), Polignac, and Bourbon-Parma brought those dynasties patrilineally to the thrones of Russia, Monaco, and Luxembourg, respectively. The Dutch royal house has, at different times, been a cadet branch of Mecklenburg and Lippe(-Biesterfeld). In the Commonwealth realms, Prince Philip, Duke of Edinburgh, and his male-line descendants are cadet members of the house of Glücksburg.
By contrast, it was also sometimes possible for cadet branches to sink in status, either due to diminished fortune or genealogical distance from the reigning line. Such was the case of the Capetian branch of the princes de Courtenay, the last male of which died in 1730 without ever having been recognized by the French crown as dynastic princes du sang despite their undisputed but remote male-line descent from Louis VI of France. Likewise, the principi di Ottajano, an extant branch of the House of Medici eligible to inherit the grand duchy of Tuscany when the last male of the senior branch died in 1737, were bypassed by the intervention of Europe's major powers, which allocated the title elsewhere. Although the Romanovs mounted Russia's throne in 1613 due to kinship-by-marriage to a tsar (Ivan the Terrible) descended from the 9th century founding ruler Rurik, when in 1880 Tsar Alexander II wed Catherine Dolgorukov, a Rurikid princess, the marriage and its progeny were deemed morganatic.
Notable cadet branches
- House of York; descendants of the fourth son of Edward III Plantagenet, King of England, who, in the course of the Wars of the Roses (1455-1485), displaced the agnatically senior line of Plantagenets, the Lancaster branch, on the English throne (1461), only to be finally displaced themselves by a Lancastrian cognatic descendant, Henry Tudor, Earl of Richmond, who obtained the crown by conquest from Richard III (August 1485). As Henry VII, he took as queen consort the heiress of the cadet branch, Elizabeth of York, in January 1486. Their son, Henry VIII, thus united in his person and on the throne of England both branches of the Plantagenets, while inaugurating the House of Tudor which would rule England until 1603.
- House of Schleswig-Holstein-Sonderburg-Glücksburg; descendants of a younger son of King Christian III of Denmark (of the House of Oldenburg), who eventually became monarchs of Denmark, Norway and Greece and of which Charles, Prince of Wales, is patrilineally a member.
- House of Bourbon; descendants of a younger son of Louis IX of France who, in the person of Henry IV of France inherited the throne of France from the senior Capetian line of the Valois in 1589; and from which sprang the Bourbon kings of Spain (including the Carlist and French legitimist lines), the kings of the Two Sicilies, and the sovereign Dukes of Parma, who currently reign in the Grand Duchy of Luxembourg in a cadet line. Also from Louis XIII de Bourbon descends the cadet branch known as the House of Orléans, to which the Citizen-king Louis-Philippe, the Orleanist claimants to the throne of France (Henri, comte de Paris, duc de France) belong, as does the House of Orleans-Braganza.
- House of Guise; Although the Dukes of Lorraine exercised continental independence, nominally they were vassals of the Holy Roman Emperors and their geo-political importance resided less in the size of their realm than in their crucial location between the competing French and German nations. A younger brother of Duke Antoine, Claude of Lorraine, was appanaged with the lordship of Guise in France and betook himself to the French court in search of his fortune. There, he was granted the title Duke of Guise as a member of the Peerage of France, he and his male-line descendants henceforth being accorded the rank of prince étranger. As the Protestant Reformation threatened the unity of France the conspicuous loyalty of Claude's descendants to the Roman Catholic Church, combined with their barely concealed ambition upon the throne of the last Valois kings, infused the Guises with unequalled power in French politics. Their role in Paris and France's wars extended their influence in European affairs, until the accession of the House of Bourbon to the throne in 1593, far beyond that of their senior cousins reigning in Nancy.
- Spencer: the comital branch of the Spencer family descended from John Spencer, the youngest son of Charles Spencer, 3rd Earl of Sunderland and Lady Anne Churchill. The couple's eldest son Robert inherited his father's title of Earl of Sunderland while their second son Charles inherited the title of Duke of Marlborough upon the death of his aunt Henrietta. When Robert, 4th Earl of Sunderland died without an heir, his titles passed to his brother Charles, 3rd Duke of Marlborough; with it the Marlboroughs (later known as the Spencer-Churchills) became the senior branch of the Spencer family. John's only son, also named John, became the 1st Earl Spencer. From the line of the Earls Spencer descend many prominent figures, including Diana, Princess of Wales, whose son Prince William, Duke of Cambridge was born heir eventual to the Crown of the United Kingdom.
- Wellington; Arthur Wellesley, the younger brother of Richard Wellesley, the 2nd Earl of Mornington, started his career as a protégé of his older brother. He entered the military, a traditional occupation of younger sons. From 1809 to 1814 he won a series of very significant victories, and was awarded a series of ascending titles; Baron Douro, Viscount Wellington, Earl of Wellington, Marquess of Wellington and, finally, Duke of Wellington. A descendant of Baron Cowley, youngest brother of Richard Wellesley, became Earl of Cowley in the Peerage of the United Kingdom, his junior line of the family thereby also achieving a higher status than that of the Earldom of Mornington in the Peerage of Ireland.
- Poore, Benjamin Perley (1848). The Rise and Fall of Louis Philippe, Ex-king of the French: Giving a History of the French Revolution, from Its Commencement, in 1789. W.D. Ticknor & company. p. 299. Retrieved 2009-03-06.