Changing default date format at instance level

The Oracle manual says:
"You can alter the default value of NLS_DATE_FORMAT by changing its value in the initialization file and then restarting the instance, and you can alter the value during a session using an ALTER SESSION SET NLS_DATE_FORMAT command. "

Which initialization file are they referring to? How does the entry in the initialization file read for a default entry of 'YYYY-MM-DD'?

I put the alter session line into glogin.sql and it works, but it is lost on a 'connect different_user' in the same sqlplus session.

-- 
Doug Edmunds
Vancouver, Washington USA
edmunds::at::pacifier::dot::com
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