How does Automatic Tranny choose?

[GeekGuyAndy]

I just reread the thread about EOC in autos and had this question... How does an a/t choose which gear to go into when shifting from N to D at speed?

I normally don't EOC over 45mph, but there are many times that I would coast in N at 55-60 for a downhill slope then shift back into D when the speed lowers. As long as I give a little rev to 2000rpm before shifting, there is no odd noises or feelings noticed. I assume that it's just throwing the tranny into 4th gear, but how does it know what gear to put it in? I have no knowledge of how trannies work, but it doesn't seem like it tries 1st gear and says "nope, let's try second" on soforth.