Does Sharing a Great-Grandmother Prohibit Cousins from Marrying

One of my pet peeves is anonymous queries to AskOliveTree. I figure if someone is writing to ask for some help or advice, the least they can do is provide their first name. So, against my usual "rule" of ignoring any requests without a name attached, I'm going to answer this one which was was not signed:

Maybe you can help me... my question to you is that I met a cousin by facebook and I we like each other Im starting to like him. My grandma is halfsister with his grandma they are from the same mother but different dads so we only share a great grandma so I would like to know if is ok if we have a relationship and can we have kids?

AskOliveTree answer: It depends. Different countries have different laws regarding degrees of cousinship and legal marriages. Different religions have different rules about who can marry.

Your relationship with your cousin is considered Canon Degree 3 and Civil Degree VI (6).  This is probably not a problem at all, but you should find out what laws your country or religion have regarding Canon or Civil Degrees of relationships and marriage.

Canon Degree is simply the furthest number of generations back to a common ancestor (in this case it is 3 - parents, grandparents and great-grandmother)

Civil Degree adds both individuals generational numbers back to that common ancestor. For you and your cousin it is 3 each, giving you Civil Degree of 6.