===All in-law marriages===
Sometimes two siblings in one family marry siblings or cousins in another family. This strengthens the ties between the two families more than a single marriage but does not in itself bring any genetic implications. It can include affinal marriages such as a man marrying his dead wife's sister, but most such marriages are not properly described as affinal and indeed there is no common name for this. They usually occur in pairs but sometimes happen in groups of three or more.
[[File:royal-in-law-marriages.png|thumb|right|350px|In-law marriages in royal tree (click to enlarge)]]