A double-second-cousin prescriptive-marriage system:



The second-most-common arrangements (among those we are considering here) are that either there are three patriclans, and the men of each patriclan alternate, by generation, which of the other two patriclans to take brides from; or there are three matriclans, and the women of each matriclan alternate, by generation, which of the other two matriclans to take bridegrooms from.

For instance:
Suppose a man H is seeking a bride.
He can’t marry someone from his own patriclan such as his sister Z or his daughter D.
In this particular system he also can’t marry someone from his mother M’s patriclan, because his father F already did that.
But it is his turn to marry someone from his grandmother (his father’s mother) FM’s patriclan, like his grandfather FF (his father’s father) did.
His FM, her brother his FMB, that brother’s son H’s FMBS, and that relative’s daughter FMBSD, all come from his (H’s) FM’s patriclan.
If his intended bride W is to come from his generation, she is probably (classified as) his second-cousin FMBSD.

If we have W=FMBSD from H’s point-of-view, then from W’s point-of-view we have H=FFZSS.
That is, W is marrying (a man classified as) her father’s father’s sister’s son’s son.

Or consider the case where there are three matriclans, and a woman W is seeking a bridegroom.
She can’t marry someone from her own matriclan such as her brother B nor her son S.
In this particular system she also can’t marry someone from her father F’s matriclan, because her mother M already did that.
But it is her turn to marry someone from her grandfather (her mother’s father) MF’s matriclan, like her grandmother MM (her mother’s mother) did.
Her MF, his sister W’s MFZ, that sister’s daughter W’s MFZD, and that relative’s son MFZDS, all come from W’s MF’s matriclan.
If her intended bridegroom H is to come from W’s generation, he is probably (classified as) her second-cousin MFZDS.

If we have H=MFZDS from W’s point-of-view, then from H’s point-of-view we have W=MMBDD.
That is, H is marrying (a woman man classified as) his mother’s mother’s brother’s daughter’s daughter.

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Although I don’t happen to know of any particular real-life case, it would be reasonable to expect that there are probably several real-life examples of both of these happening simultaneously.

That is; there are probably examples of cultures and societies in which;
A man must marry a woman who is (classified as) both his FMBSD second-cousin and his MMBDD second-cousin;
and a woman must marry a man who is (classified as) both her MFZDS second-cousin and her FFZSS second-cousin.

(Note each of the bridegroom’s grandmothers is (classified as) a sister of one of the bride’s grandfathers, and each of the bride’s grandfathers is (classified as) a brother of one of the bridegroom’s grandmothers!)

....

In other words everyone marries a double-second-cousin; someone whose coefficient-of-relatedness would be 6.25% (1/16), as close as a first-cousin-once-removed or a half-first-cousin, if the relationship is what we’d call “actual” instead of “classificatory”.
And it can be “actual”; but it doesn’t have to be.

....

In the scientific study of consanguinity among humans, a mating between a man and a woman who are as closely related as full-second-cousins — that is, coefficient of relatedness r >= 3.125% (1/32) — are regarded as consanguineous, but any more distant relationship is not regarded as consanguineous.
For instance, second-cousins-once-removed, and half-second-cousins (having only one greatgrandparent in common), and double-third-cousins, all have r = 1.5625% (1/64), and are (therefore) not (regarded as) consanguineous.