doubly related. Double first cousins share both sets of grandparents. Half cousins are descended from half siblings and would share one grandparent. The children of two half siblings are first half cousins. If half siblings have children with another pair of half siblings, the resulting children would be double half first cousins. While there is no agreed upon term, it is possible for cousins to share three grandparents if a pair of half siblings had children with a pair of full siblings. Non-blood relations Step-cousins are either stepchildren of an individual's aunt or uncle, nieces and nephews of one's step-parent, or the children of one's parent's step-sibling. A cousin-in-law is the cousin of a person's spouse or the spouse of a person's cousin. A child of a sibling of an aunt or uncle by marriage would also be a cousin-in-law. In the Basic family tree example David and Edward are cousins-in-law. None of these relationships have consanguinity. Consanguinity Consanguinity is a measure of how closely individuals are related to each other. It is measured by the coefficient of relationship. Below, when discussing the coefficient of relationship, we assume the subject and the relative are related only through the kinship term. A coefficient of one represents the relationship one has with oneself. Consanguinity decreases by half for every generation of separation from the most recent common ancestor, as there are two parents for each child. When there is more than one common ancestor, the consanguinity between each ancestor is added together to get the final result. Between first cousins, there are two shared ancestors each with four generations of separation, up and down the family tree: ; their consanguinity is one-eighth. For each additional removal of the cousin relationship, consanguinity is reduced by half, as the generations of separation increase by one. For each additional degree of the cousin relationship, consanguinity is reduced by a quarter as the generations of separation increase by one on both sides. Half cousins have half the consanguinity of ordinary cousins as they have half the common ancestors (i.e. one vs two). Double cousins have twice the consanguinity of ordinary cousins as they have twice the number of common ancestors (i.e. four vs two). Double first cousins share the same consanguinity as half-siblings. Likewise, double half cousins share the same consanguinity as first cousins as they both have two common ancestors. If there are half-siblings on