> They did a lot of mathematical analysis to decide that 80 bits for the internal number format would result in accurate 64 bit results. Something about losing lots of bits of accuracy during exponentiation, so you want extra bits the size of the exponent.

I'm not sure if it was the reason, but 80-bit intermediate values lets you compute pow(x,y) as exp(y*log(x)) to full precision for the full range of 64-bit floats.