Clearly Rule 20.3 applied to Kelpie after she started to move astern by backing her main, and it required her to keep clear of any other boat that was not also moving astern by backing a sail. Rule 20.3 is in Section D of the Part 2 rules, and the Section D preamble states, "When [Rule 20.3] applies between two boats, Section A rules do not." Therefore, Interloper had right of way over Kelpie at all times while Kelpie was moving astern, regardless of whether Interloper was on port tack, starboard tack, or tacking. Two other rules applied-Rules 14 and 16.1. As you can see from the diagram, when Kelpie began to back down Interloper was on port tack. While Kelpie was gathering sternway Interloper tacked and assumed a collision course with Kelpie. As soon as Kelpie's crew saw the course that Interloper had assumed, they made every effort to avoid contact, but were unsuccessful.