The first deacde had ten years;
1, 2, 3, 4, 5, 6, 7, 8, 9, 10 AD !
c/f centuries;
1,2,...,99,100 1st
101,102,...,199,200 2nd
etc
This, the *tenth* decade of the century, began in
1991.
I realise this contradicts the usage we ALL make, with
the "90's" beginning in 1990, like calling year's the
first of the "1800's","2000's" etc.
A little thing I'd never considered before :)
I realise isn't neat or intuitive, it's just what's necessary to keep the numbering consistent with a calendar invented before they used "0".
hmmm..
mike