Cousins are people who share ancestors and thus likely some DNA. Often, we are most interested in the Most Recent Common Ancestor (or MRCA) that the cousins share.

The count up to the common grandparents gives the “depth” of the cousin. First cousins, that we are all used to knowing and having, share common grandparents but not parents. Each one has a parent that is a sibling to the others’ parent. Second cousins share common great-grandparents. So, for each of them, one of their parents is a cousin to the others’ parent. Carrying this further, 3rd cousins share 2x Great Grandparents. Or algebraically, nth cousins share (n-1)x Great Grandparents.

But what happens when the number of generations to the common grandparents is different between the two lines. Then you figure out the maximum nth cousin level that gets to one of the two. And you add that number of “removed” that represents the further generational steps to the other. So you are related to your grandparents 1st cousin as a 1st cousin, twice removed. Your grandparents 1st cousins grand kids are your 3rd cousins. A diagram may help here.

Program-matically, because computer scientists start counting or include zero, your siblings are your 0th cousins.

For most, they share two MRCA’s as the two grandparents had multiple children that led to the different ancestral lines the cousins each have. Sometimes, only one grandparent is in common as two different partners/spouses generated the different children. Technically, and in doing a generation difference and consanguinity count, this adds a step in the process and would introduce the term “half”. But, most often, we only apply the term “half” to siblings and maybe first cousins. After that, the importance of only sharing one MRCA at the same generation level has traditionally not been considered. But with genetic genealogy it becomes more important to be precise about this.

See Randy’s old page on Family Distance for an explanation until we flesh this out further. We need to rewrite that to simplify the diagram and text.