IM4Travel ,
I have gotten comps from UA, US, CO, AA, and DL. In all cases, all they wanted to see was my status on the airline that I was asking them to match status on. As you can see, I have not gotten an NW comp, but would bet that NW does not run things differently in this regard. You typically can just fax your web account summary showing status, or even just a copy of your card (I've done both), and never been asked anything about it, etc. Frequent flyer programs are more business/marketing tools to drive share to said carrier, as opposed to vehicles for awarding you or me for being loyal, etc. Said is the key reason the airlines have them, and the comp is a cheap way for carriers to possibly drive some share of business to their airline. In fact, I have read that UA in particular will comp you over and over and over again. However, most other airlines at least say publically that they generally will only do one comp.....however have not tested multipple comps on one carrier yet.