This from the topic "Consanguinity in Canon Law" in "New Advent Catholic Encyclopedia"
Paragraphs headed "Mode of Calculation"
"In calculating the degree of consanguinity, special attention must be paid to three things, the line, the degree and the stock or root. The stock or root is the common ancestor ... from whom descend as from the nearest common bond the persons whose blood-relationship to be determined .... The blood relationships computed according to the distance from the stock whence it is derived and this is the rule by which degrees or steps on consanguinity are determined."
It then mentions similarities and differences between Roman civil law and canon law and different ways of computing degrees of relationships.
"But the Canon Law, in the collateral line of consanguinity, computes for marriage one series only of generations, and if the series are unequal, only the longer one. If the two series are equal, the distance is the number of degrees from the common stock. Thus brother and sister are in the first degree, first cousins in the second degree; uncle and niece in the second because the niece is 2 degrees from grandfather, who is the common stock."