It's the difference between "the beginning" and "the end" - to begin a divorce, one files a Petition for Dissolution and after completing the necessary procedural requirements, one files a Judgment for Dissolution to end the divorce - after either a default (the other side doesn't respond), a written settlement, or a trial.
It is not correct that you can get divorced six months after filing - it has to be 6 months after service providing...thru a 3rd party.... a copy of the Petition, along with a Summons,etc., to your spouse. And note that this doesn't happen automatically - steps must be taken by you to get the Judgment. Go to the self-help center at the court.
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