This was mentioned last year; I see that the FAQ has an excellent article
on the subject, although the link to Hepworth is dead.
I've just disinterred an article from its box, so had a search. There are
[at least] 2 good sources (although they're really the same content), one
of which is a live link to Hepworth:

http://www.hep2o.co.uk/bititesguideearth.htm

and, as a PDF:

http://uk.wrs.yahoo.com/_ylt=A1f4cfb...cfm%3ftype=pdf

or:

http://bit.ly/aA8CUq

(downloads PDF of 83kB).
--
Peter.
2x4 - thick plank; 4x4 - two of 'em.