In theory at least, SOIC is half the pin pitch of DIP and SSOP is half again. Thus SSOP is 0.1/4 = 0.025" or 0.635mm. Except I see many parts, from TI and Linear especially, which are SSOP (or TSSOP) with 0.65mm pin pitches. Which is slightly annoying.
Unfortunately, after say 18 pins in a row, the difference starts to add up so one has to be careful when adding footprints in Eagle.
The question is: was there ever really a standard for these parts or are manufacturers free really to do what they want? Or is this a metric vs imperial thing. There are 0.5mm TSSOP parts of course but they are more obvious.