A relationship between two persons can be determined if a common ancestor (eg. parents, grandparents, etc.) is known. To determine the relationship, first put the name of the common ancestor (ie. one whom the two persons are directly descended from) into the box at the top left of the table below.

Then, going to the right of the common ancestor, put one of the two persons into the appropriate generation that he/she is from the common ancestor (eg. if the person is a great great grandchild to the common ancestor, then the person is in the fourth generation from the common ancestor, thus the person goes into the Gen4 box that's labeled 2G GChild).

Next, going down from the common ancestor, put the other of the two persons into the appropriate generation that he/she is from the common ancestor (in the same manner as did for the first person).

Last, follow down the generation of the first person and follow right the generation of the second person to the intersecting box. The label in that intersecting box shows how the two persons are related.