plumb line
noun
1
                    
                                          
              
          
                                                      : a line (as of cord) that has at one end a weight (such as a plumb bob) and is used especially to determine verticality                                      
                
                    
2
                    
                                          
              
          
                                                      : a line directed to the center of gravity of the earth : a vertical line                                      
                
                    
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  Merriam-Webster unabridged




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