I've been doing some research through many topics in both the civil and the technical forums, and I gathered as much information as I could on the subject of flight deck commonality. My purpose with posting all this information is that if any of you detects mistakes, you can let us all know, so that we have no more doubts on this issue. Here I go:
The classic A300 has no commonality with the A310. However, the newer A300-600 shares type rating with the 310.
One type rating for the A318/319/320/321.
One type rating (pretty cool, huh?). Only aircraft types with different number of engines and one same rating.
The A320 family and the A330/A340 DON'T share type rating. Nevertheless, transitioning to each other is very quick and not expensive.
OK this is not too simple. There are three different "families", each with its own cockpit. There's the 737-200, the 737-300/-400/-500, and the 737NG. I had always thought each family had its own type rating, but now I know that it's pretty much up to the airline. The best example is Southwest. WN pilots hold one type rating for all their 737s, which means they can fly every -200, -300, -500 and -700 in the fleet. Another example is Continental. One type rating for the -300, -500, -700 and -800. However, the -900 is in a different group for them.
One type rating
One type rating for all 757s and ALL 767s. For example, Continental pilots hold one rating for the 757-200, 757-300, 767-200 AND 767-400. They can fly Honolulu-Houston in the 767-400 one day and Houston-Newark in the 757-200 the next day. The exception here I believe is Delta, they have the 767-400 as an independent type.
One type rating.
The 767-400 and the 777 DON'T have the same type rating, but transitioning from the 767-400 to the 777 is much easier than transitioning from a 767-200 or -300.
One type rating for series 10, 20, 30, 40 and 50. The DC9-81/-82/-83 have a different type rating, which can be shared with the MD87/88 with differences training. The MD90 type rating can be shared with the MD88's, like in Delta's case. I believe some US Airways senior pilots could fly the DC9s AND the MD80s. (They don't have DC9s anymore, right?).
Remember, let us all know if you find any mistake here.