Douglas DC-9 & McDonnell-Douglas MD-80/MD-90 series
The DC-9 was the start of a long series of aircraft, strongly varying in length and also engine types, ultimately ending with the MD-90. It was the first twin jet airliner in the west with a T-tail and engines attached to the rear fuselage. Its nose is based on the DC-8, without the air intakes below the nose but with a single eyebrow window on each side. Other features are small, slender wings, a double bubble fuselage cross section and an air intake at the base of the vertical stabiliser.
How to recognise the different versions of the DC-9, MD-80 and MD-90 will be added later.
Confusion possible with
This aircraft started its life as MD-95 so it is obvious that you can confuse it with a DC-9, MD-87 or maybe another from the family. The Boeing 717 is best recognised by the shape of the engine nacelles. It also has the flat tail cone of the later MD-80 series and an MD-87 like tail.
This Chinese regional jet also looks like a small DC-9 or MD-87, because the MD-80 and MD-90 were built in license in China. Likely a lot of manufacturing tooling was reused. Most obvious differences are the disappearance of the eyebrow windows and the high bypass engines on the ARJ21.
Especially the shortest versions of the DC-9 could be confused with the F28, as both have an eyebrow window. However, the Fokkers have a blunt tail cone, that can be opened as air brake, and a large dorsal fin.