Some slide rules ( `` duplex '' models ) have scales on both sides of the rule and slide strip , others on one side of the outer strips and both sides of the slide strip , still others on one side only ( `` simplex '' rules ) . Some slide rules ( `` duplex '' models ) have scales on both sides of the rule and slide strip , others on one side of the outer strips and both sides of the slide strip ( which can usually be pulled out , flipped over and reinserted for convenience ) , still others on one side only ( `` simplex '' rules ) .