The system of laws and justice of the Seven Kingdoms are largely defined by its feudal system of local government.
Justice belongs to the Iron Throne. Lords are allowed to pronounce justice in the name of the king. Lords have the right of pit and gallows over their own lands, i.e. having the authority to hang people or arrest them and have them according to the king's law. Landed knights cannot exercise the same right without the leave of their liege lord.
On the king's small council, the master of laws serves as one of the king's councilors. The office of master of laws is often combined with the office of justiciar. The royal executioner is known as the King's Justice. The King's Justice holds the responsibility for the dungeons of the Red Keep in King's Landing. During the Targaryen reign, the Lord Confessor was responsible for questioning prisoners. Although the last Lord Confessor served under King Daeron II Targaryen, a room reserved for the man serving in the office remains in the tower of the dungeons.
During his Conquest, King Aegon I Targaryen appointed a master of laws on his council. After his Conquest, he decided to keep the laws of each of the six conquered kingdoms as they had been before his Conquest. Instead, while he traveled through his realm on his progresses, Aegon brought six maesters with him to advise him on the local laws and traditions. In 55 AC, King Jaehaerys I Targaryen began work to create the first unified code of law for the six kingdoms he ruled over, codifying, organizing, and reforming all the laws of the kingdom. In this task he was assisted by Septon Barth, Grand Maester Benifer, Lord Albin Massey, and Queen Alysanne Targaryen. Gathering, organizing, and studying the existing laws alone already took them two years. The reforms in total would take decades.
During the first a hundred and eighty-seven years of Targaryen rule, Dorne remained an independent kingdom. When Dorne was finally unified under the Iron Throne one of the conditions was that "Dornish law would always rule in Dorne." As per the agreement between King Daeron II Targaryen and Prince Maron Martell, the lords of Dorne were allowed significant rights and privileges that the other great houses of the Targaryen kingdom did not. In addition to the use of their royal title and the autonomy to maintain their own laws, the Dornish kept the right to assess and gather the taxes they owed the Iron Throne, with the Red Keep having irregular oversight. Other concessions made were similar, causing dissatisfaction among the other nobles of the realm.
King's Peace
The first law King Aegon I Targaryen enacted was the "King's Peace". The law requires petty lords and landed knights to take their disputes to their liege lord, and abide by his judgement, while disputes between great houses were adjudicated by the Crown.
Rule of six
The "rule of six" was established by Queen Rhaenys Targaryen during the first decade of Targaryen rule over the Seven Kingdoms. While holding court, she was presented with a man who had beaten his wife to death. The brothers of the deceased women wished for her husband to be punished for his crime, but the husband insisted that he had been within his rights. He had caught his wife abed with another man, and in accordance with the law was thus allowed to strike her with a rod no thicker than a thumb (the "rule of thumb"). However, the woman’s brothers claimed that the husband had struck his wife a hundred times. Queen Rhaenys and her maesters agreed that only six blows were lawful; One for each of the Seven gods, except the Stranger, as the woman had given offense to the gods with her adultery. The other ninety-four blows, on the other hand, were counted as unlawful. From that day onwards, the "rule of six" was counted as part of the common law.
Rule of thumb
The "rule of thumb" restricts the size of the rod used by husbands to punish their adulterous wives. By law, the rod should not be thicker than a thumb.
King Maegor's laws
King Maegor's laws, or "Maegor's laws", is a term used by Queen Regent Cersei Lannister and the High Sparrow to refer to the laws enacted during the reign of King Maegor I Targaryen during the Faith Militant uprising. As the Faith of the Seven continued to resist his rule, King Maegor raised a set of laws which forbade holy men from carrying arms. Later on, Maegor added to his earlier laws by placing bounties on the heads of those faithful who remained defiant: a gold dragon was given for the scalp of Warrior's Sons and a silver stag for the scalp of a Poor Fellow.
Queen Alysanne's laws
The term "Queen Alysanne's laws" is used to refer to several laws enacted during the reign of King Jaehaerys I Targaryen. According to Archmaester Gyldayn, the usage of the term is both sloppy and incorrect, as Alysanne had no authority to enact laws, issue decrees, make proclamations, or pass sentences.
Widow's Law
Queen Alysanne Targaryen accompanied King Jaehaerys I Targaryen for part of his progress through the Vale in 52 AC, during which time she held her women's courts at Gulltown and the Gates of the Moon. She brought what she learned during these courts to King Jaehaerys, which resulted in the enactment of the Widow's Law.
The Widow's Law reaffirms the right of the eldest son (or eldest daughter, is there are no sons) to inherit. However, the law requires the heirs to maintain their father's surviving widow, no matter whether she had been a second, third, or even later wife, under the same conditions as she had been before her husband’s death. The widows could no longer be driven from their late husband’s castle, deprived of her servants or possessions, or her income. The law similarly prevented men from disinheriting the children from an earlier marriage in favor of children from a later marriage.
Abolishment of the right to the first night
In 58 AC, after having held her women's courts at several places in the North, Queen Alysanne went before the small council at King’s Landing to speak with them on the horrors the women experienced under the lord's right to the first night. With the support of Septon Barth, Alysanne convinced Jaehaerys to abolish the right to the first night. Henceforth, any lord who still continued to practice the first night would be found guilty of rape.
Age of Majority Law
During the Council of Regents prior to the coming of age of Aegon III Targaryen, the regency council along with the Grand Maester and the Maesters of the Citatel determined that the socially accepted structure for Age of Majority was insufficient, research finding that pregnancies in women were less frequently fatal when the woman is at least eighteen years of age and that men possessed the capability to make more measured decisions when of a similar age. As a result, the Age of Majority for both men and women was raised to eighteen years of age, encompassing independent rule and consumation of marital bonds. ((While not a canon law, this law was added into the lore to maintain compliance with Linden ToS in a way that made sense with the genre.))
Trials of the Crown
Trials, at least among the nobility, often begin with a prayer from a septon beseeching the Father Above to guide them towards justice. The accused and witnesses are sworn to honesty before he gives testimony at a trial. The accused is allowed to be present as witnesses give their testimony, in front of one or multiple judges.
Trials of the Faith
Of old, the Faith of the Seven tried and judged those found guilty of a crime themselves. Per the agreement between the Faith and King Jaehaerys I Targaryen, the Faith agreed to accept justice from the Iron Throne in exchange for the Iron Throne always defending and protecting the Faith. These agreements led to the official abolishment of the Faith Militant, and ended the Faith Militant uprising.
Trial by combat
Any knight accused of a wrongdoing is allowed by law to demand a trial by combat. The right to a trial by combat also extends to nobles who are not knighted. The accused and accusers are allowed to have champions fight in their place. When a person is killed in a trial by combat, by law it is not considered murder.
A more ancient custom, though seldom used, is a trial of seven, in which seven men fight on the side of the accusing party, and seven on the side of the accused party.
When the person who stands accused is royalty, their champion has to be a knight of the Kingsguard. When the accuser is royalty, however, they are within their rights to select a champion who is not a sworn member of the Kingsguard.
Laws of inheritance
The laws of inheritance in the Seven Kingdoms are not clear cut. According to George R. R. Martin, these laws are “modelled on those in real medieval history... which is to say, they were vague, uncodified, subject to varying interpretations, and often contradictory”. Male-preference primogeniture is customary, but not binding, for most nobles. A man's eldest son is his heir, followed by his second son, then his third son, and so on. In theory, the youngest son is followed in the line of succession by the eldest daughter, after whom come her sisters in birth order.A man’s daughter inherits before her father’s brother. However, a lord also has the option of naming one of his younger sons heir, passing over his elder children, or to name the child of another as his heir. When there is no clear heir, claims can be presented to the King. The only exception is Dorne, where no distinction is made between sons and daughters. Instead, children inherit in order of birth regardless of gender, as per Rhoynish custom. When a ruling lord dies and leaves no clear heir, his widow might lay claim upon his lands and rule until her own death (e.g., Lady Donella Hornwood and Lady Barbrey Dustin), and in such a case, might name an heir by herself.
The role of legitimised bastards throughout the Seven Kingdoms is also unclear i.e., whether they follow trueborn children, or join the line of succession in order of birth as if they had been trueborn all along.Legitimisation, once made, is irreversible. While unlegitimised bastards have no legal claim, they may still threaten legitimate descendants' inheritance. The illegitimate Jon Snow's decision to join the Night's Watch pleases Catelyn Stark, because in accordance with the vows he is thus required to take, Jon will never father children who might contest her grandchildren's inheritance of Winterfell.
The right of succession may be renounced. Right of succession is also lost when someone becomes a member of the Night's Watch, a septon, a maester, or joins the Kingsguard. Traitors may be attainted, in which case even his descendants would lose their right to succeed.
Taxes
In the Seven Kingdoms, taxes are collected locally. Lords pay taxes to the crown. The Great Houses gather the taxes from their region.The Great Houses subsequently pay their taxes to the Crown.An exception are taxes owed to the Night’s Watch by the villages and holdfasts located in the New Gift, which are paid in kind, not in gold.
Lords can have treasurers in their service to handle the incoming taxes.
Captivity
Those accused of a crime can be arrested and kept in a dungeon. Many, if not most, castle have their own dungeons. In the Red Keep at King’s Landing are the black cells. The cells and dungeons at the Red Keep are located in a squat, half-round tower. The top floor holds cells for prisoners kept in a degree of comfort, such as knights or lordlings who might be ransomed, while the entrance to the dungeons sits on the ground floor. The dungeons, built by King Maegor I Targaryen, have four levels. The uppermost level has cells with high narrow windows where common criminals are confined together. The second level has smaller, personal cells without windows for highborn captives. Torches in the halls cast light through the bars. The third level cells, the black cells, are smaller still and have doors of wood so that no light enters them. The lowest level is used for torture. The Red Keep’s dungeons are the responsibility of the King's Justice.
The "dungeons" of the Arryns at the Eyrie are called the sky cells. These cells are located six hundred feet in the sky, and have only three walls and a stooping floor. The cells are known to drive men mad.
Cities (like Gulltown)and castles (including Winterfell, Riverrun, Harrenhal,Rosby, Dragonstone, the Nightfort at the Wall,the Twins.) have their own dungeons.
Crimes and punishments
Thievery and pick-pocketing
Poaching
Outlawry
Rape
Slavery
Free folk crossing the Wall
Night's Watch
Execution
Miscellaneous