What does the compiler option -W (upper case) do?

["Dr. David Kirkby" <david.kirkby@xxxxxxxxxx>]

I've got some code written by someone else, which has

g++ -W -Wall foobar.cc

gcc is not indicting -W is an unknown option. The gcc documentation says -w 
(lower case) is to suppress all warnings, but I can't find -W (upper case) 
documented.

Dave