I remember reading somewhere that according to formal SI rules double
prefixes should not be used, unless a single prefix is not available.
Thus pF is OK but uuF is not OK. I used to say nano-pico-second,
referring to 10^-21 s, I must now say zepto-second.
The prefix zepto is for 10^-21 and the prefix zetta is for 10^+21.
I learned this at the last AAPT meeting. Double prefixes, going by a
factor of 1000 each time, are allowed outside zepto and zetta. For
example femptozepto (fz) for 10^-36 and terazetta (TZ) for 10^+33.
The smallest is zeptozepto (zz) and the largest is zettazetta (ZZ).
Now you can really say you understand physics.
For more details contact the AAPT Committee on SI Units. I am not
a member of that body but I was present at their meeting.