Newbie... I need help.

I don't have anyone to ask at the moment, and I was trying to get some answers on the Net, and found this forum.
I am trying to understand percentiles and deciles. There's not much about it in my book, so I asked some of my friends, but they seem to be uncertain about this just as much as I am.
Anyway... my question is this:
in my book it's stated that the 1st decile starts with the 1st percentile, and the 2nd one with 20th percentile and so on.

What I don't get is, if the median is in the 5th decile, why does the book say that the 2nd decile starts with the 20th percentile, because according to that, 5th decile starts with the 40th percentile, and the 6th decile starts with the 50th. but apparenty the median is not in the 6th decile, but in the fifth (??)

I would really appreciate the explanation :)
please help :(

i still haven't figured this out and i'm sure that there is someone here for whom this is a piece of cake and can help me faster than it took me to post this...

so, please, please, please... help me out :)
percentile and decile

The first decile starts with the 1st percentile, the second decile starts with the 11th percentile, and so on. The fifth decile begins with the 41st percentile and ends with the 50 percentile. Does that help?