I looked into my companion certificates from DL AMEX for this year, and realized that under conditions...

"Eligibility - Eligible on Delta, Delta Connection carrier (except American Eagle, BigSky and ExpressJet flight number series 7750 - 7829), certain Northwest flights operated under the Delta code and Delta Shuttle flights only (excludes all other Delta-designated codeshare flights). "

So I can use my companion certs. when I fly NW? cool... should I check availability on the award calendar? (sorry, had to add that one)