Example 1: Military personnel position themselves for the Saber Arch ceremony. The soldiers are standing along two  lines parallel to a straight line coming down the center of the airplane staircase. They are also all standing the same distance from that center line. The soldiers are forming a locus of points. 
                           Set of points: the soldiers' locations 
                             Condition: Saber Arch ceremony 
                             Locus: 2 lines parallel to the center line, 
                             and an equal distance from center line 
                              | 
                            This 2007 photo shows Secretary of Defense Gates, his wife and former President Bush being welcomed in Texas.  | 
                         
                         
                         
                           Example 2: Fifteen years ago, a series of trees were planted along a straight line driveway leading to an estate.  Each tree was planted the same distance from the center of the driveway. 
                             Set of points: the trees' locations 
                               Condition: as a driveway border 
                               Locus: 2 lines parallel to the center line, 
                           and an equal distance from center line  | 
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