Your first cousin is a child of your aunt or uncle. You share one set of grandparents with your first cousin, but do not have the same parents.
Your second cousin is the grandchild of your great-aunt or great-uncle. You share one set of great-grandparents with your second cousin, but do not have the same grandparents.
Third, Fourth, and Fifth Cousins
Your third cousin is the great-grandchild of your great-great-aunt or great-great-uncle. You share a set of great-great-grandparents with your third cousin, but do not have the same great-grandparents. Fourth cousins have one set of great-great-great-grandparents, but not the same great-great-grandparents. And so on.
Cousins of different generations is explained by using the word “removed”. Saying that cousins are “once removed” means that there is a difference of one generation between cousins. For example, the first cousin of your father is your first cousin, once removed. In this case, your father’s first cousin is one generation younger than your grandparents and you are two generations younger than your grandparents. This one-generation difference is explained by saying that your are cousins “once removed.”
Twice removed means that there is a two-generation difference between cousins. If you are two generations younger than the first cousin of your grandparent, then the relationship between you and your grandparent’s first cousin are first cousins, twice removed.
A cousin who is the child of one’s mother’s brother or one’s father’s sister.
A cousin who is the child of one’s mother’s sister or one’s father’s brother. Parallel cousins are the children of two brothers or two sisters.
If two siblings in one family marry two siblings from another family and each couple has a child, the children are double first cousins. The addition of the word double to the first cousin term is because because they share the same four grandparents. Regular first cousins share only one set of common grandparents, while double first cousins share both sets of grandparents plus all lineal and collateral relatives.