Conception: "Don't Open A Weak 2-bid with a side 4-card major"
The Truth: Preempts can be made with a variety of shapes.
Players love rules. Bridge teachers sometimes give in to the temptation to give players lots of them. I can't tell you how many bad things I hear after a sentence starts, "My Teacher told me ..."
The truth is that preempts are entirely too complicated to describe with rules. None of the following are good rules: "Don't
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AQJ2 
K103 
AK 
Q832