Merge To Center merges
            any selected vertices so they become shared and positions the resulting
            shared vertex at the center of the original selection. Any faces
            and edges associated with the originally selected vertices also become
            merged as a result.
         
         NoteThe 
Merge
                  to Center feature merges to a central point based on
               the center of the current selection. If you want to collapse edges
               on a component by component basis, select the edges, then select 
                        Edit Mesh > Collapse,
               or shift + right-click and choose 
Merge/Collapse Edges
                  > Collapse Edge from the marking menu.