If I'm reading the OP correctly... it's airline-101. It's called "Point Beyond", at least according to NW:

http://www.nwa.com/plan/comm10.html

Point Beyond Policy

Fares apply for travel only between the points for which they are published. Tickets may not be issued at fares published to and/or from a more distant point than the points being traveled. When a passenger enplanes at an intermediate point the Northwest agent can verify that the passenger actually flew on the previous flight. If Northwest finds that the passenger did not travel on the previous flight, Northwest reserves the right to deny transportation to the passenger unless the difference between the fare paid and the applicable fare for the actual itinerary is collected.

Join the club. At one time, DTW-BWI was $1000, but TOL-DTW-BWI was $450 (and for those that don't know, TOL is about 50 miles south of DTW). Drive 100 total miles to save $550. But it is sure tempting to just hop on the flight in DTW, right?

I'm sure back-to-back is going to come up sooner or later (with the SIN 6-day layover requirement), but that URL also explains that, too.

Steve B.