A Double Rule of Three

He thought he saw a Garden-Door
 That opened with a key:
He looked again, and found it was
 A Double Rule of Three:
‘And all its mystery,’ he said,
 ‘Is clear as day to me!’

Lewis Carroll, Sylvie and Bruno and Sylvie and Bruno Concluded

 
After going in the wrong direction for a while, I understood that this is not about applying the Bellman’s Rule twice. I think that Lewis Carroll (like Charles T. Brooks) quite probably referred to cross multiplication.Rule of Three

Then again, I am not shure whether my direction was that wrong. Carroll could have used Double Rule of Three with a double meaning:
※ the extension to the rule of three for cross-multiplication and
※ the double application of the Bellman’s rule.

More on the Rule of Three: Alfred Crowquill (pen name of Alfred Henry Forrester), Comic Arithmetic, London 1843, p. 96

 
2022-05-31, updated: 2022-06-12   (MG007)