A cadet branch is a noble House that descends from another noble House.
They are usually created when a younger member of a noble House, who is not the current heir of the family seat, is granted lands and titles of his own. Bastards, if legitimized, or trueborn descendants of legitimized bastards, when granted lands and titles of their own, also found cadet branches.
Larger and wealthier noble Houses such as the Lannisters or Tyrells may have numerous younger cousins, but these are all part of the main House - albeit far down in the line of succession. When younger children or cousins form their own cadet branch, they become head of this new House, though the cadet branch is not considered as prestigious as the main line.
Strictly speaking, a "cadet branch" consists of only male-line descendants, in an unbroken chain from father to son - in which case the founding member was a younger son of another noble House. The inheritance systems in the Seven Kingdoms do not put as much value on descent through the female line. New Houses have been formed from the female-line younger descendants of other Houses, but their lineage is considered inferior compared to other cadet branches who can claim male-line descent. For example, House Tyrell descents from the Gardeners, but only through a Gardener daughter - which means that other houses of the Reach descend from a Gardener son have a superior line of descent compared to the Tyrells. The Tyrells are not, therefore, technically a "cadet" branch.
Given that Dorne, uniquely among the Seven Kingdoms, practices equal primogeniture it probably makes no distinction between cadet branches founded by younger sons or younger daughters.
Known cadet branches through male-line descent
- House Blackfyre, founded by Daemon Blackfyre, a legitimized bastard son of King Aegon IV Targaryen. The house became extinct after being defeated in numerous rebellions attempting to claim the Iron Throne.
- House Karstark of Karhold, formerly House Stark of Karlon's Hold. Founded by Karlon Stark, a younger son of House Stark of Winterfell. Over time the "Karlon's Hold Starks" became known as the "Karstarks".
- House Lannister of Lannisport. Descended from House Lannister of Casterly Rock, they rule the city of Lannisport as vassals of the main Lannister line.
- House Florent. Descended from House Gardener of Highgarden.
- House Baratheon. Descended from House Targaryen.
- House Baratheon of King's Landing. Founded by Robert Baratheon after taking King's Landing at the end of Robert's Rebellion. It is actually a cadet branch of House Lannister in all but name, as Robert's "trueborn" children were really fathered by Ser Jaime Lannister, the Queen's twin brother.
- House Baratheon of Dragonstone. Founded by Stannis Baratheon when he is made Lord of Dragonstone after Robert's Rebellion.
- Given that Robert secretly had no trueborn heirs, and his youngest brother Renly had no issue, following Robert and Renly's deaths this branch technically became the de facto "House Baratheon" - particularly because Stannis is Robert's younger brother, not a distantly removed relative. Nonetheless the Lannisters continue to insist that Cersei's children are Robert's heirs.
Known cadet branches through female-line descent
- House Tyrell, descending from House Gardener through the female line.
- House Lannister of Casterly Rock, descending from the (so-far unidentifed) house of Lann the Clever through the female line. One of Lann's female First Men descendants was married by a male member of the Andal family who invaded the region six thousand years ago, to secure their conquest.
- (As revealed in the World of Ice and Fire sourcebook (2014), Lann's descendants actually were called "Lannister" during the time of the First Men: when an Andal adventurer married the daughter of the lord of Casterly Rock, he continued to use the name of his wife's House for their children - "Lannister" - because it was more prestigious.)
- House Baratheon, descendants of House Durrandon, whose last member Argella Durrandon married Orys Baratheon. Their connection is shown in their heraldry, as Orys Baratheon took House Durrandon's sigil and words as his own.
- It was also strongly rumored that Orys was Aegon I Targaryen's bastard half-brother, but given that Orys was never officially acknowledged, the Baratheons are not legally a male-line cadet branch of the Targaryens, despite their Targaryen bloodline. Regardless, Robert, Stannis, and Renly did have Targaryen blood through their mother (according to the Histories & Lore featurettes for the TV continuity, apparently different from the novels, in which their paternal grandmother was a Targaryen).