English: To rearrange the letters ABC, systematically make three "choices", with the zeroth choice the decision between the three options for the first element. The "first" choice is to choose the second element, and there are 2 choices for each of those three initial choices (for the first element.) The second choice involves the second element, but since one element is already chosen, this introduces two choices. For example if A was selected to be the first letter, the second letter can be only B or C. For the third "choice" you have only one option. For example, once AC is selected, you must select B to create the ACB permutation.