prohibited degrees

the relationships between persons that make it illegal for them to marry. In England the position is regulated by the Matrimonial Causes Act 1971. For a man, the relationships are his mother, daughter, grandmother, granddaughter, sister, aunt, niece, the wife of his father, son, grandfather or grandson. For a woman, the prohibited relationships are her father, son, grandfather, grandson, brother, uncle, nephew, the husband of her mother, daughter, grandmother or granddaughter, her husband's father, son, grandfather or grandson. The general prohibitions were relaxed slightly by the Marriage (Prohibited Degrees of Relationship) Act 1986, which permits marriage between a man and a woman who is the daughter or granddaughter of a former spouse of his if both the parties are over 21 and the younger party had not, before attaining the age of 18, been a child of the family.

In Scotland the law is similar, being set out in the Marriage (Scotland) Act 1977, as amended by later legislation including the 1986 Act (above).