Consanguinity means descent from a common ancestor. Laws about who may marry, and old church rules about dispensations, are written in terms of degrees of it. The trouble is that lawyers have counted degrees in two different ways.
Two ways to count
Civil law counts every step: up from one person to the ancestor, then down to the other. Siblings are 2 degrees apart, and first cousins are 4. In Kinage's terms the civil degree is a plus b. The older canon-law method, used by the Catholic Church before 1983, counted only the longer branch, so siblings were in the first degree and first cousins in the second. Kinage shows both numbers for every term, and the chart lays them out.
What the law says
Almost every legal system forbids marriage between parents and children and between siblings, and many extend this to grandparents, aunts, uncles, nieces and nephews. First-cousin marriage is legal in the United Kingdom and many other countries. In the United States it varies state by state, and some states prohibit it entirely. Some jurisdictions also bar marriage within affinity, for instance to a former parent-in-law.
What this site is not
Kinage works out a relationship name and its degrees. It does not tell you whether you may marry, inherit, adopt or sue. Laws differ by country and change over time, and exceptions exist for religious and customary marriage. For a real decision, check the current law where you live or ask a solicitor.
The first cousin page shows its degree under both counting methods.