What does "commensurable" mean?

Commensurable: Having a common measure; capable of being exactly measured by the same number, quantity, or measure. -- Com*men"su*ra*ble*ness, n. Commensurable numbers or quantities (Math.), those that can be exactly expressed by some common unit; thus a foot and yard are commensurable, since both can be expressed in terms of an inch, one being 12 inches, the other 36 inches. -- Numbers, or Quantities, commensurable in power, those whose squares are commensurable.