When comparing the descent of two people who wished to marry each other, it was necessary to investigate their ancestry as far back as their great-great-grandparents. If an ancestor was common to both, they would require a dispensation before marrying.
Now if only every mediaeval Pope had a laptop and access to this simple Consanguinity Test. It is a simple spreadsheet with narrowing cells in each column, much like a sporting knockout format in reverse. For example, the Lumley-Conyers marriage of 1489 required a dispensation and is summarised in the Ricardian 2016 (pp. 113-120 with the diagram on p.118).
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