There are two main types of relationship used in genealogy (there is also adoption, but this is not considered further here):
- consanguinity, or blood relation, is where two individuals are descended from a common ancestor - i.e. there is a genetic link
- affinity is where the relationship depends on a marriage - i.e. it is the relation which each party to a marriage has to the relatives of the other
Related by blood
Lineal consanguinity (sometimes referred to as 'direct line') is where one of two individuals is descended from the other - such as between a grandson and grandfather.
Collateral consanguinity is where two individuals share a common ancestor but one is not descended from the other - such as siblings, cousins, nephews/nieces and uncles/aunts.
Direct (lineal) relations who are 2 generations apart have the prefix 'grand' (as in grandfather, grandson, etc). Nephews/nieces and uncles/aunts who are 2 generations apart may be prefixed with 'great' or 'grand'. Relations who are a greater number of generations apart are prefixed by 'great' repeated as required (usually abbreviated as e.g. '3x great grandfather').
1st cousins have the same grandparent. 2nd cousins have the same great grandparent, and so on. Where two individuals share a common ancestor but are a different number of generations apart from that common ancestor, then the difference is appended to the relationship as a number of times removed (usually abbreviated as e.g. '3x removed').
There are also some special cases:
- half-siblings share only a single parent (i.e. where that parent has remarried) (it is in principle also possible to have half-uncles and half-aunts - i.e. half-siblings of a parent - but these terms are rarely used)
- double cousins - the children of 2 brothers who have married 2 sisters, or of a brother and sister who have married another sister and brother - double cousins share all lineal and collateral relatives and share 25% of their genetic material
Note that step relatives (such as a stepbrother or stepfather) are not related by blood and are therefore covered under Related by marriage below.
Related by marriage
The simplest types of relation by marriage are signified by adding 'in law' - for example, father in law, daughter in law. However, the spouse of an aunt or uncle (and conversely of a nephew or niece) is instead prefixed with 'husband of' or 'wife of'.
Step relatives are where a parent remarries. For example, a stepdaughter is a spouse's daughter by a previous marriage, while a stepmother is the wife of one's father by a subsequent marriage and a stepbrother is is the son of one's father's or mother's spouse by a previous or subsequent marriage. It is also possible in principle to describe someone as a stepuncle or stepaunt - i.e. the sibling of one's stepparent - but these are rarely used. Unlike half siblings, step relatives are not related by blood.