Geneology
The 737-100 came first, and was upgraded to the -200 almost immediately. Darn few -100's sold.
Then came the classics: the -300 came first, bigger jet, bigger engines, etc. Then the stretch model, the -400. Then the bright idea of a smaller version, compatible with the -200 in terms of size, and lighter/more fuel efficient than the -300, but same basic technology. Thus the -500. (All of the classics could have round dials, as SWA jets do, or the 4-tube EFIS, as many other airlines have.)
Then came the NG's, and they made the numbers increase with size: -600, -700, and -800 going from "small" to "standard" to "x-large." The -600 is the NG successor to the -500, and they're roughly the same size (except wingspan), just as the -700 is to the -300, and the -800 to the -400.
For SWA, the smaller jets are sort of a "gotcha" waiting to happen -- you sell 137 seats, and a -500 shows up at the gate, and you have an immediate oversell. Ooops!
Still, a -600 would be fun to fly -- that much power in a light airframe with all the NG bells & whistles! If only they would land like the -200 does (by far the best touchdowns of all the varients we fly)!
Snoopy
