Many sailors and PC-members have trouble with this one. But if we forgo appendices C en D, it only appears in the rulebook in two rules: RRS 17 & 18. How can it be so difficult? Perhaps because it is a very subjective definition, each boat has her own opinion about "Proper course"
Proper Course A course a boat would sail to finish as soon as possible in the absence of the other boats referred to in the rule using the term. A boat has no proper course before her starting signal.
Two boats start a race and each one wants to finish first. But they have totally different ideas on which side of the course is preferable. Each has a reasonable explanation why they want to go that way. Both boats sail there proper course.
Two overlapped boats on a broad reach catch up to a group in front. The leeward boat makes a judgement that he can pass the group the quickest, if he goes to windward of them. So she luffs and heads up to do that. The fact that there's a windward boat, who perhaps thinks that passing the group to leeward must be quicker, doesn't negate that for the leeward boat going windward is the proper course. She would have chosen the same course if the windward boat wasn't there.
In short, there can be several proper courses on any given moment. You can't predict beforehand which of them will be quicker. As long as there's a logical reason for choosing and you choose fairly consistently, any PC will accept that as a proper course.
And even after arriving at the next mark one is clearly in front of the other does not invalidate one of the chosen paths. It still is a proper course.
Let’s have a look at rule 17.1, where there's a restriction on the right of way boat, not to sail above her proper course. This rule has three specific conditions, which all have to be met before this rule applies. The right of way boat must have created a leeward overlap coming from clear astern, the boats must be on the same tack and finally the distance between the two boats must be no more than two lengths. If all these three conditions are met then - and only then - the restriction, not to sail above a proper course, is applicable.
These three conditions can also be met, when the windward boat is still tacking (not yet on a close hauled course). The rule excludes the restriction in that case.
If rule 17.1 is applicable does it change the r.o.w situation? No, it does not. The keep clear boat still has to keep clear. Even if the r.o.w. sails above her proper course and infringes RRS 17.1, the windward boat still has to keep clear. If she doesn't, she breaks a rule (RRS 11). Many committees will DSQ both in that case.
In rule 18.4 the r-o-w boat has also a restriction on where she can sail. If her proper course would be to gybe, she must do so, as soon as she can sailing around the mark.
Because boats haven't started yet and therefore cannot finish as soon as possible, there's no proper course before the starting signal. A boat from clear astern, creating a leeward overlap is subject to 17.1, but she can luff up to head-to-wind before the signal. However, as soon as the start gun has been fired, she must bear away to a close-hauled course.
I have no doubt that this definition will be discussed in many situations in the future. If you have a specific incident, don’t hesitate to send it in.