Here’s a question… how do we think about the last sentence of rule 18.1?  Could one argue that Red gave Green the MR she was entitled to but Green did not take it?

The last sentence of 18.1 doesn’t say when mark-room was taken … it says when MR is given.  

1. Room to sail to the mark? 
2. Room to leave the mark on its proper side?
3. Room to round the mark as necessary to sail the course?

• Does rule 18 apply?  Which rules of 18, when, why?
• Does rule 18 continue to apply at 4?
• Does rule 21 apply? (Is the boat taking a penalty). 
• If Green is still entitled to MR at 4, is she sailing within the MR she is entitled to (rule 44.1(b) applies)?

• Boats are overlapped.
• G has passed head to wind in the zone, so, in accordanc  with  RRS 18.2( d), RRS 18.2 (b) and (c) do not apply, so  RRS 18.2 (a) applies.
• G is between R and the mark.
• So G is overlapped inside R and R is required by  RRS 18. 2 (a) to give G mark-room. 

Andrew … a penalty doesn’t impact mark-room as Section B and C rules still apply when rule 21 applies. 

• Mark-room includes room
• Room include space to comply with her obligations under other rules of Part 2
• Rule 21 is a rule of Part 2
• Greens obligation under rule 21.2 is to keep clear of a boat not taking a penalty (Red)

1. Room to sail to the mark? 
2. Room to leave the mark on its proper side?
3. Room to round the mark as necessary to sail the course?

1. Green’s original corridor to the mark remain from when she entered the zone? or
2. Green gets a new calculation of space to sail promptly to the mark from position #3 when her MR transitions from 18.2(b) to 18.2(a)? or
3. Green’s space to sail promptly to the mark is being continuously recalculated between positions #1-#4 until such time that Red and Green are close enough to each other that they are influencing one or both boat’s course?

1. Calculated once when she entered the zone?
2. Recalculated once when the part of rule 18 providing MR changes?
3. Recalculated continuously until the Red is close enough that they are influencing each other’s course?

 Red has to give mark room to green, who must immediately start to sail within her corridor (18.2(a)) 

The mark-room that P was required to give S was the space S needed in the existing conditions to sail promptly to the mark in a seamanlike way. That space was a direct corridor from S1 to a position close to and alongside the
mark on the required side. P gave S that room. However, because S had right of way she was not required to remain within that corridor; she was permitted to sail any course provided that she complied with rules 16.1 and 18.4.

1. Outside the MR she is entitled to, to sail [promptly]  to the mark on its proper side, because that corridor was set when Green entered the zone at 11 on the dial, or
2. Inside the MR she is entitled to, to sail [promptly] to the mark on its proper side, because …
   1. Green’s corridor reset when her MR changed from 18.2(b) to 18.2(a), or
   2. Green’s corridor didn’t set at all until position 4, when Red is close enough to influence Green and Red’s course?

Green is sailing … (2) inside the MR she is entitled to, to sail [promptly] to the mark on its proper side, because … (1) Green’s corridor reset [at pos 3+] when her MR changed from 18.2(b) to 18.2(a),

Here’s a question… how do we think about the last sentence of rule 18.1?  Could one argue that Red gave Green the MR she was entitled to but Green did not take it?
 
 The last sentence of 18.1 doesn’t say when mark-room was taken … it says when MR is given. 

I think this room continues until she has passed the mark as necessary to sail the course.  It is clear that she has not yet done this, so   mark room has not yet been given. 

I think the simple answer to Phillip's question, above, is that, assuming Yellow entered the zone clear ahead, she is entitled to mark-room at position 2 and, assuming yellow has passed the mark at position 3, Blue has mark-room at that time.
 
 This transfer of obligations was unintended by the rule-writers. I know this because I was one of them.
 
 The brief history of the words "or has been given that mark-room" in rule 18..2.(d) is this:
 
 There used to be a criterion for when rule 18 turns off -- rule 18 applied "until the boats have passed the mark". But that turns out not to work; picture two Sunfish, with their long booms, passing close by a windward mark, overlapped and with the windward boat bows-out on the leeward boat. The leeward boat cannot release her sheet and bear off until she is past the mark, and at that point the windward boat is also past the mark; yet we surely don't want to turn off rule 18 yet, as the boats have barely begun to round the mark! The simple requirement "the boats have passed the mark" also doesn't work in other common scenarios, especially where there is a current and boats must sail beyond the point where the mark is clear astern of them, in order to sail the course without risk of hitting the mark. So that statement was removed in 2009. It was not replaced because, as John Allen points out above, such a clause seems unnecessary -- rule 18's provisions about mark-room "turn off" (or, more precisely, become moot) when there is no more room to be given.
 
 Alas, a rule that turns on at a clear point (when one of the boats enters the zone) and then doesn't turn itself off again bothers the heck out of people, especially because rule 18.2(b) says that a boat clear ahead or overlapped at the zone is entitled to mark-room "thereafter", which is a pretty strong word; it's used in religion to indicate eternity. Anybody who presented a class or seminar on the rules between 2009 and 2013 knows that sooner or later somebody would ask, "But when does the rule turn off?" The explanation, that it really doesn't turn off until both boats leave the zone, but that doesn't matter because at some point there is no more room to be given, takes a long time and satisfies nobody.
 
 So we decided to answer the question if we could find an easy fix. Rule 18.2(d) already had two conditions where a boat can lose her right to mark-room, so that seemed an obvious place to add an additional condition. Further, it's rule 18.2(b) that uses the word "thereafter", and as everybody knows, it's rule 18.2(b) that almost always grants mark-room, so a rule that talks about entitlement for mark-room under rule 18.2(b) seemed like a good place to put a criterion for that rule turning off.
 
 Rules-makers soon recognized that this was a mistake. We never intended to switch the right to mark-room, from the boat clear ahead or overlapped inside at the zone, to the other boat. However, as a practical matter it's not a big deal -- once a boat has been given mark-room, she is sailing the next leg (or finished) and the requirement for her to give mark-room to a boat overlapped inside her is not onerous. Furthermore, almost anything she might do that deprives the other boat of the space to round the mark is prohibited by rules 15 or 16 (remember, it is unseamanlike to hit a mark, so if she changes course in such a way that the boat inside her is compelled to hit the mark, she breaks rule 16, even if she doesn't owe that boat mark-room).
 
 Still, the problem should be fixed, and in their annual conference in October, 2018, World Sailing passed changes to rule 18 and the definition "mark-room", to answer the question better in 2021. These changes remove the words "when mark-room has been given" from rule 18.2(d), and move them to a new sentence in rule 18.1 turning off all of rule 18 when mark-room has been given (regardless of whether it was given under rule 18.2(b) or some other part of the rule) and the mark no longer influences the course of the boat entitled to that room. WS also passed changes to the definition mark-room making it clear that mark-room has been given only when the boat entitled to mark-room is past the mark and she can continue to sail the course without hitting that mark. This is not an urgent change and will not come into effect until January, 2021. Note also that the change made in 2018 can be reviewed by National Authorities and edits proposed in 2019; in fact, the US Racing Rules Committee is currently working on some minor word-smithing for the new definition. 

[assuming Green entered zone at 11 o’clock, at pos 4] Is Green sailing …

1. Outside the MR she is entitled to, to sail [promptly]  to the mark on its proper side, because that corridor was set when Green entered the zone at 11 on the dial, or
2. Inside the MR she is entitled to, to sail [promptly] to the mark on its proper side, because …
   1. Green’s corridor reset when her MR changed from 18.2(b) to 18.2(a), or
   2. Green’s corridor didn’t set at all until position 4, when Red is close enough to influence Green and Red’s course?

1) Outside the MR she is entitled to, to sail [promptly] to the mark on its proper side, because that corridor was set when Green entered the zone at 11 on the dial

2A) Inside the MR she is entitled to, to sail [promptly] to the mark on its proper side, because … Green’s corridor reset [was calculated anew] when her MR changed from 18.2(b) to 18.2(a), 

2B) Inside the MR she is entitled to, to sail [promptly] to the mark on its proper side, because…..Green’s corridor didn’t set at all until position 4, when Red is close enough to influence Green and Red’s course?