The Rule of Seven was created and implemented separately by two individuals. One is Mr. Robert Berthe of France, who is a published author of bridge book(s).
Mr. Gerald Fox of Napa, California, bridge expert, teacher, and author, also independently devised the Rule of Seven.
These two bridge players were wondering if it would be preferable for the declarer to hold up an Ace. After establishing the question, they set about finding the answer. They arrived at the following conclusion.
If the declarer subtracts from 7 (seven) the total number of cards in the suit in his own hand and the dummy, the result is the number of times declarer should hold up with his Ace.
An example always helps. Declarer has A98 and the dummy has 42. Together declarer and dummy have 5 cards in this particular suit. Declarer subtracts the number 5 from the number 7, and 2 (two times) is the number of times declarer should hold up with his Ace.