A fourth cousin is someone in a family who shares the same great-great-great-grandparents. First cousins share the same grandparents and second cousins have the same great-grandparents. Ordinal numbers of cousins are in the same generation as each other. For instance, all of the children of siblings are first cousins.
Cousins who have children are different degrees of "removed." For example, the children of a first cousin are considered a first cousin once removed. The term removed in this case refers to being one generation removed from another person. The first cousin of a grandmother is technically considered a first cousin twice removed because there is a two-generation difference.
Generations of cousins enumerate two less than a common ancestor. If a shared ancestor is 10 generations in the past, the person is an eighth cousin in the same generation. The son or daughter of a seventh cousin is a seventh cousin once removed.
One of the easiest ways to document family members is with a relationship chart. This handy visual aid allows two people to determine common ancestry. The chart connects rows and columns between the ancestor the two people share. Simply trace a horizontal line from one relationship on the chart to a vertical line with the other relationship to find out how two people are related.