rule

noun

• A regulation, law, guideline.

  "All participants must adhere to the rules."

• A ruler; device for measuring, a straightedge, a measure.
• A straight line (continuous mark, as made by a pen or the like), especially one lying across a paper as a guide for writing.
• A regulating principle.
• The act of ruling; administration of law; government; empire; authority; control.
• A normal condition or state of affairs.

  "As a rule, our senior editors are serious-minded."

• Conduct; behaviour.
• An order regulating the practice of the courts, or an order made between parties to an action or a suit.
• A determinate method prescribed for performing any operation and producing a certain result.

  "a rule for extracting the cube root"

• A thin plate of brass or other metal, of the same height as the type, and used for printing lines, as between columns on the same page, or in tabular work.