I read part of this in the primer but would like to slightly clarify the question by giving concrete examples. How do they determine if a ticket is Nested or it is Back to Back. What constitutes an airport in a similar location.
Here are my examples. Ticket (A) and (B)

Scenario 1
(A)YVR-SEA-SFO
(B) SFO-SEA-YYJ
(B) YYJ-SEA-SFO
(A) SFO-YVR

YVR and YYJ are only 40 miles apart

Scenario 2
(A) YYJ-SFO
(B) SFO-SEA-YYJ
(B) YYJ-SEA-SFO
(A) SFO-YVR

(same as number 1, but no similar transit airports)

Scenario 3
(A) CLM � SEA-SFO
(B) SFO-SEA-YYJ
(B) YYJ-SEA-SFO
(A) SFO-SEA-CLM

Two end points YYJ and CLM are in different countries, but again only 20 or so miles apart


1) My understanding is that they are nested if the two turn around cities are far enough apart. How far is far enough apart?

2) How is a lay person supposed to know as I almost did this a couple of years ago before I heard about flyertalk in a legitimate run (had to escort grandma to SFO, get back to YYJ and then get back to SFO to take her back to YVR?
.

3) I was told back to back is fine if I used two different carriers (in this case AC & AS for example) What about if I used NW and AS??

This would be ticket A with ticket B nested, a valid use of both tickets.

This would an open-jaw ticket A (YYJ-SFO-YVR) with ticket B (which returns to the point of origin) nested inside. If the usage of ticket B circumvents the minimum stay requirements of ticket A, then it is considered a back-to-back and would violate the carrier's contract of carriage. Transit cities do not matter.

Quote:

<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Scenario 3
(A) CLM � SEA-SFO
(B) SFO-SEA-YYJ
(B) YYJ-SEA-SFO
(A) SFO-SEA-CLM

This is ticket A with ticket B nested inside. This nesting is valid because ticket B does not return to the point of origin.

They are nested if you use ticket B within ticket A. They are back-to-back if they return to the point of origin and circumvent minimum stay requirements. Two cities are considered the same point if they are co-terminals (which YVR and YYJ are not, YYJ and CLM are not, but YVR and YXX may be, SFO and OAK may be - depending on the airline/fare).

Quote:

<font face="Verdana, Arial, Helvetica, sans-serif" size="2">2) How is a lay person supposed to know as I almost did this a couple of years ago before I heard about flyertalk in a legitimate run (had to escort grandma to SFO, get back to YYJ and then get back to SFO to take her back to YVR?</font>

You would have been expected to buy either one ticket with all segments in one reservation (therefore, if turnaround in SFO was minimal, you would have been forced upon a higher fare).

Alternatively, you could have also been expected to purchase/use your tickets in sequential order. (i.e. YYJ-SFO-YYJ on ticket A with minimal turnaround) and the YYJ-SFO-YVR with minimal turnaound).

In both cases, a higher fare would almost certainly be the outcome.

Quote:

<font face="Verdana, Arial, Helvetica, sans-serif" size="2">3) I was told back to back is fine if I used two different carriers (in this case AC & AS for example) What about if I used NW and AS??

Not a problem. If you purchase tickets on two different carriers, you should not be violating either carrier's contract of carriage. I don't think whether they are partners matters.