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How do cross-cousins differ from parallel cousins?

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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

In the study of kinship, scholars distinguish between types of first cousins based on the gender of the parents who are siblings.12

Parallel cousins (sometimes called ortho-cousins) are the children of a father’s brother or a mother’s sister—that is, the children of same-sex siblings. In many cultures, these individuals are categorized similarly to siblings.12
Cross-cousins are the children of a father’s sister or a mother’s brother—the children of opposite-sex siblings.12

While the English language uses the term "cousin" for both, many societies maintain a strict distinction between them, particularly regarding marriage practices. In many cultures, cross-cousin marriage is preferred or even obligatory to strengthen clan alliances, whereas parallel-cousin marriage is frequently prohibited by incest taboos because parallel cousins are often considered part of the same immediate family unit. However, some societies, such as those in the Middle East, may prioritize parallel-cousin marriage to keep wealth and family ties within the same lineage.12