One time on Usenet, Melba's Jammin' said:

OK, when melting chocolate or using a double boiler for just about
anything else, there is always the admonition to set the top pan *above*
the water level in the bottom pan. Why can't that top pan touch the
boiling (or simmering or hot or whatever temp) water? Is it going to
be THAT much hotter on the meltable if it does?

Yes, it is. You don't want the upper pan as hot as the water, just
as hot as the heat coming off of the water. Chocolate is very easy
to burn...

--
Jani in WA