Rule 18 applies at all
: all windward marks, all leeward marks (including the marks of a gate), marks at the end of reaching legs, and at all finishing marks -- except of course when it doesn't (see RRS 18 - Part 1 - When RRS 18 Applies
And RRS provides for how shall be given. Simply stated, RRS (a) provides that when boats are overlapped the outside boat shall give the inside boat mark-room... However remember that (a) always applies unless rule (b) applies. But (b) covers about 90% of mark-room questions so (a) is generally the fall back rule and (b) is controlling. So start by deciding if RRS 18 applies. If it does then decide if RRS 18.2(b) applies. If not, then apply RRS 18.2(a).
But, before trying to understand (b), it is good to remember that while (b) applies, at least three other rules apply; a basic right-of-way rule (e.g.; RRS ), avoiding contact (RRS ) and exoneration (RRS ). And in some situations, one or more of RRS , , , and apply. Got it? Don't worry, it's complex (and gets much worse), but the basic rule is fairly easy to grasp and we all use it every time we race.
So here is the basic analysis: In Diagram 1, Yellow and Blue are approaching the first windward mark on starboard. At position 1, both boats are required to leave the to port and B is in the , meeting all the requirements of the first sentence of RRS . And none of the exceptions apply as it is not a starting mark and they are not approaching the mark to start; they are not on opposite tacks; the proper course of neither boat is to tack; both boats are approaching the mark and the mark is not a continuing obstruction. We of course don't do all of this analysis in our head at the time (unless you are an umpire), but as competitors we generally recognize the situation developing as we approach the mark. The inside boat inevitably asks for "mark-room" and the outside boat complies.
So we conclude that mark-room is required both by custom and by the rule. However, is it RRS 18.2(a) or 18.2(b) that requires Y to give mark-room to B? RRS (b) states that If boats are when the first of them reaches the , the outside boat at that moment shall thereafter give the inside boat . Y is clearly overlapped with B when B enters the zone. It appears RRS 18.2(b) applies, so 18.2(a) does not.
Thus, B gets mark-room from Y and RRS (d) requires that Y give B mark-room until "has been given." And just a precursor to the later discussion of 18.2(d), B still gets mark-room at position 4, but probably not at position 5. We'll discuss this particular rule later, but let's conclude that B no longer gets mark-room from Y at position 5.
Thus, B gets from the time she enters the at position 1 until position 5. Y is still obligated under RRS to of B as long as she the windward boat. And both boats are obligated by RRS to avoid contact if possible. In addition, B will be exonerated under RRS should she break RRS when she luffs at position 3.
This is an example of a pretty simple mark-rounding we each see repeatedly every time we race. And while we tend not break any rule in this situation, the way the rule applies is not without some complexity. We will get into more complex situations (that will probably only be of interest to judges and umpires), but the general rule presented here is how to think about the basic mark-room rules as you approach that first very important windward mark.