Comment # 20 on bug 929471 from Dr. Werner Fink

The static hostname should be without any domain name.  Simply because it is a
local hostname which is normally not related to any network device.  Also in
Europe where very long domain names are common, it could be that if the domain
name become part of the static hostname, it might be overflow the array used
for the hostname (currently the limit is 64 characters, on older systems it
might be 32 characters).

Compare with man:proc(5)

       /proc/sys/kernel/domainname and /proc/sys/kernel/hostname
              can be used to set the NIS/YP domainname and the hostname of
              your box in exactly the same way as the  commands
              domainname(1) and hostname(1), that is:

                  # echo 'darkstar' > /proc/sys/kernel/hostname
                  # echo 'mydomain' > /proc/sys/kernel/domainname

              has the same effect as

                  # hostname 'darkstar'
                  # domainname 'mydomain'

              Note,  however,  that  the classic darkstar.frop.org has the
              hostname "darkstar" and DNS (Internet Domain Name Server) 
              domainname "frop.org", not to be confused with the NIS
              (Network Information  Service)  or  YP  (Yellow Pages)
              domainname.  These two domain names are in general different.
              For a detailed discussion see the hostname(1) man page.