RC's covered the technical stuff...but here's a simple spotter's guide:

For DC-9's, they started small, with the -10 series (subvariants like the -14 or -15 being the same length, but having other differences), and then got longer.

The -10's look very stubby, not much bigger than an RJ. Most of the -10's have a single overwing exit, and none of them have leading edge devices, so you don't see the drooping leading edge of the wing.

-30's are several rows longer, and are the majority of the fleet. All have dual overwing exits and leading edge devices on the wings.

The -40's are longer (and the -50's slightly longer yet) and can be distinguished from a -30 by a small strake (s tiny little stub fin that's about 24" long but only about 4" deep) on the lower portion of the fuselage just behind the cockpit.

I don't know of a good way to tell between a -40 and a -50.

And the astute will note that there is actually a -20 series, which is the length of the -10 but has the leading edge devices from the -30. But NW has none of them (only 10 were built, and I think only 2 or 3 are in service now).

Steve

[This message has been edited by sllevin (edited Feb 22, 2004).]