> Last digit means how many 0's to add. 222 is 2200 pf and 220 there
> is no 0 to add means 22 pf. If it was 221 then would be 220 pf

That's been my experience. If the manufacturer means something else
then they generally indicate that by adding more information. For example
I have small ceramics, marked '330pF', '47p', '10n' and just '2.2'. Some
older, larger mica caps even have '100,000pF'. I suspect they would have
come from the days of paper and wax 'condensers' in valve circuits

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