In my junior high school years (grades seven and eight), we had excellent metalworking and woodworking shops. I can't recall the man's name, but one of the woodworking teacher's pet peeves was the use of the word 'ruler' for 'rule'. Whenever a student would call a 'rule' a 'ruler', the teacher would point out to him that, "The Queen of England is a ruler; the measuring tool is a rule."
LittleMachineShop.com has a good article on steel rules and their use. Note that they consistently abstain from use of the word 'ruler' -- the tool is called a 'rule'.
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