Nested tickets are allowed. Back to Back tickets are not. However, splitting it up between 2 airlines just means that it's unlikely you'll get caught; it doesn't make it ok with the airlines. Delta would rather get the high price fares from people who can't/won't do back to back ticketing and the partial revenue from those who do than get less revenue by allowing back to backs.

So my understanding is that the COC even covers travel with another airline.

I see why they have the rule, but I am glad not to be in a too tempting situation to use this method.

Ethics can be a pain, especially when the airlines do things that benefit themselves to our detriment.

EYE for an EYE???????

I sure would like to see where or how the airlines can regulate the nesting/or back to back if you are using another means of transportation (airline or bus or train, etc)

Though they do have it in the COC, or so I am told for their OWN airline, I doubt it could exist for other modes of travel.

DL's COC doesn't affect your travel on other carriers. Besides, like you said, they can't regulate them.

What they can do is invalidate your DL ticket if they find out you bought a ticket on another carrier to circumvent Delta's fare rules.

They don't care if you're breaking NW's COC with a back-to-back. That's NW's problem. Of course, with NW, it's the same thing -- you violate NW's COC with your NW ticket but not DL's.

Put the two together, and you have two "illegal" tickets.

As to whether this applies to travel back to the origin other than restricted roundtrip fares on an airline (e.g., unrestricted air fare, train, bus, car, etc.), I don't know. Interpreting the COC literally, it does say "two or more restricted round trip fares". The spirit of the COC might include all travel, not just restricted air fares, but that's getting a little ridiculous in the interpretation IMHO.

Wow, and we haven't even gotten into the ethics yet, just trying to interpret the fare rule and get the names of the schemes right.

The restriction on "returning to your point of origin via another RT fare" ticketing (gee, is that name sufficiently unambiguous yet?) is still pretty hard to draw a line on.

So assume that BOS-XXX and XXX-BOS RTs on the same airline are out. BOS-XXX and XXX-BOS on different airlines, we've discussed that too.

How about BOS-XXX with an XXX-PVD round trip in between? BOS and PVD aren't co-terminals, after all. Or MHT-XXX and XXX-PVD, even.

Or, say, you've been scheduled for a conference at XXX, and the company paid for your BOS-XXX ticket. Then the conference is rescheduled to only be half of the original scheduled time, so you buy an XXX-BOS ticket so you can come home and do laundry....

We were supposed to get de-regulation, but what we actually got was even worse, self-regulation!