What are the difference between a seventh son and the seventh son of a seventh son?

7th son of 7th sonA seventh son is the 7th son born to a couple. (all 7 have same mother and father)

A seventh son of a seventh son is the 7th son born to a couple and whose father (male half of above mentioned couple) was the 7th son born to his father.

Diagram:

7th son of 7th son

Father&Mother

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1st 2nd 3rd 4th 5th 6th 7th

son son son son son son son

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1st 2nd 3rd 4th 5th 6th 7th

son son son son son son son

PS. periods are there simply to keep text suitably separated in Diagram. They would otherwise be spaces.