Dll number - there is a standard, and if you grab the hacked up libtool from chucks site, you will automagically create .dll's to that standard.
The standard is cyg<libname>-<API revision>. ie cygpopt-2.dll API revision only changes when non-backward compatible changes occur - see the libtool docs on library numbering for more info. (ie -2 is _NOT_ the second revision of the library, and may be the 10 or 20th.) Rob