Bastardy

A bastard is a person whose parents, at the time of their birth, were not married to each other. As a polite way of referring to someone who is bastard-born, someone may be referred to as a 'natural son' or 'natural daughter.' A less polite term, indicative of the social stigma against bastards, is "baseborn." A euphemism for being bastard-born is "being born on the wrong side of the sheets." If both parents of a bastard are nobles then the child is not considered "baseborn."

Family Life and Status

In the series, it is not unexpected for noblemen to have bastard children, however, it is not typical for a noble to bring his bastards home and raise them with his own children. It's more usually expected that he will see to the child's well-being to some degree. A nobly born wife has the right to take insult at her husband's bastards being introduced into her household and being commensurate in rank with her legally born children.

Bastards whose parents are both of the nobility may be considered non-baseborn, although even a royal decree has considerable difficulty in removing the stigma of a bastard and trueborn children of a bastard might change their surnames to show their legitimate nature. For example, a legitimate son of a Waters might change their surname to Longwaters. At any point, the biological father of a bastard may acknowledge him and bring him formally into his house.

Far more often a bastard is acknowledged but not legitimized. For example, Eddard Stark acknowledged Jon Snow as his son and a member of House Stark, but did not legitimize him. Snow retained the bastard name of the North and the social status it conferred, and did not enter the line of succession of House Stark. Many bastards are never acknowledged and don't even know who their fathers are.

Rights of Inheritance
The baseborn have few rights under the law and custom when it comes to rights of inheritance. A bastard may inherit if the father has no other trueborn children nor any other likely kin to follow him. Additionally, a bastard can inherit if he is legitimized by a royal decree. A legitimized bastard traditionally falls in the order of succession at the end, after all trueborn offspring, including daughters. This has been disputed, however, such as in the case of the Blackfyre Pretenders.

Heraldic custom regarding bastards is fairly loose; bastards who take arms (noble born, knighted, etc.) often, but not always, take the coat of arms of their fathers with the colors reversed. A bar sinister is sometimes added, as exemplified by Ser Walder Rivers's sigil. A bastard that wants to emphasize his filiation and minimize his own bastardy may decide to use the same sigil as his father, as did Glendon Flowers, perhaps illegally.

However, any man can be knighted, even a bastard. A bastard may even be appointed to the Kingsguard. In the Night's Watch, any man may rise to command, no matter the circumstances of their birth, such was the case of Cotter Pyke, commander of Eastwatch-by-the-sea, and Jon Snow and Robin Hill, who became its Lord Commanders.

Surnames
Each of the nine constituent regions of the Seven Kingdoms have bastard surnames decreed by custom. Bastards with a high-born parent are given these surnames to hold them apart from their fathers' houses. Bastards with no known relation to a noble house have no surname, like other smallfolk.