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.
Best Answer
It's not because 0.635 and 0.65 are close to each other that they're meant to be the same. There are also 2.5 mm pitch connectors next to the more standard 0.1" = 2.54 mm. Metric vs imperial. They're just different things which should be treated as such.
Note that with packages going further away from DIP the imperial standard is often abandoned in favor of metric. For instance DFN and QFN are mostly 0.5 mm pitch, and other dimensions are in mm as well, like in this drawing. Dimensions for SMD in inches becomes rare.