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Why are first cousins classified as consanguineous kin of the third degree?

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consanguinity (Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above

First cousins are classified as consanguineous kin of the third degree based on the proportion of genes they share due to common ancestry.

In modern genetics, the degree of consanguinity is determined by the likelihood of individuals sharing genetic material inherited from common ancestors. While siblings and parent-child pairs are considered first-degree relatives because they share approximately half of their chromosomal constitution, the degree of relatedness decreases as the distance from the common ancestor increases.123

First cousins are categorized as third-degree kin because they share one-eighth of their genes in common. This classification reflects their biological relationship as collateral kin, sharing the same degree of genetic relatedness as a great-grandparent and a great-grandchild.12