Great post, Hibernicus! Your analogy to a knight's move has made me understand the nomenclature for the first time after struggling with it several times over the years!

It's fascinating that the "taboo" sense remains in the language even though it is absent from the culture, but I wonder what the first three "degrees of taboo" are. Siblings and parents/children would be 1 and 2, I assume, but what's #3? Grandparents?