The relationship could have been that of an uncle and nephew or of cousins. Before the use of middle names, it was not uncommon to have two or more men in a family with identical names. The older man was known as Senior and the younger as Junior. It is important for the researcher to keep in mind that a man known in his younger years as William Smith, Jr. may have been known as William Smith, Sr. after the death of the older man. First cousins share common grandparents.
Second cousins share great grandparents.
To calculate the relationship, find the closest common ancestor. Determine the number of generations that the more distant individual is further removed from the common ancestor. Genealogies which number individuals using the Modified Henry System, permit calculating the relationship directly from the individual's numbers. In the Modified Henry System, each individual's number represents his family line in the family being discussed. The number of digits is the number of generations from the first listed ancestor.