Regarding the comment that CO has access to book all NW seats (on those routes continuing from NRT to the other Asian cities that NW serves), this isn't really true.

Look at ITN and check out flights from IAH (or your city of choice) to SIN, using CO and/or NW. On most dates, you will notice that flight availability for the segment from NRT to SIN will depend on whose metal you fly trans-pacific, regardless of how the flight is ticketed. Here are some interesting examples:

- CO metal IAH-NRT, connecting to NW metal from NRT to SIN. The latter segment usually shows as sold out regardless of whether it's ticketed on the native NW flight number or the CO codeshare, even if the IAH-NRT CO flight is ticketed on the NW codeshare.

- NW IAH-NRT, connecting in MSP or DTW. The latter segment usually shows as available, even if the trans-pacific segment (or all segments) is ticketed on the CO codeshare.

The above is true for the same actual flights on the same days! So, on day X, NW 19 NRT-SIN will show J9 Y9 if it connects from a NW metal trans-pac flight, and on the very same screen, NW 19 NRT-SIN will show J0 Y0 if it connects from a CO metal trans-pac flight.

I find this to be quite strange, and have never seen anything like it before. Obviously, NW is trying to preserve seats on their onward connecting flights from NRT to the rest of Asia for passengers on their trans-pac flights. This makes sense, as they have several flights that funnel into NRT, but only one connecting flight to SIN. Then, add to this the 2 CO non-stops from the continental US, and they often have more demand than seats.

What's amazing to me is that the various CRS's have a feature that supports this variable display of inventory, depending on the connection. Also, I've confirmed that the system that CO reservations uses shows the same behavior. When I pointed it out to an agent, she first dismissed it as a difference between using the NW native flight number and the CO codeshare. But, I asked her to look closely, and she saw the same thing that ITN shows. At this point, I got the telephone equivalent of a raised eyebrow from the agent, which tells me that it seemed unusual to her as well.