Convention = consistency = usability
One of the key principles of usability is adherence to convention - if we set things up in a consistent way then it’s a whole lot easier for people to understand how something works and then make educated guesses to achieve a result. A mis-applied or incomplete consistency is often worse than inconsistency. (from Gnome site)
The Office of the Domain Name Commissioner in their publication “Making a name for yourself…”(PDF 295kb) states “Second level domains (2LD) allow .nz registrants to communicate extra information about themselves in their domain name. Unlike “yourcompany.com”, with .nz there is always a group of letters in between (most widely used are yourcompany.co.nz).” (my emphasis applied)
Excellent - we have a clear rule, providing consistency.
So why do we have www.govt.nz and www.parliament.nz ?
I wouldn’t mind so much if www.government.govt.nz redirected to www.govt.nz - but it doesn’t.
(as for www.e.govt.nz, at least www.e-government.govt.nz works!)
