The Word "So"

When the word so means "therefore," it starts a new sentence and takes a semicolon or period in front of it and no single comma after it because it is only one syllable.
...We left early; so I missed his phone call. ...The company had already fired him; so there was nothing I could do to help. ...The blue car ran the signal; so the accident was inevitable.
Absolute rule.
Happy punctuating!