What is horizontal relationship?

Horizontal relationship is a relationship between equals, as opposed to a relationship between superiors and inferiors (vertical relationship). Source: http://www.eric.ed.gov/ERICWebPortal/custom/portlets/recordDetails/detailmini.jsp?_nfpb=true&_&ERICExtSearch_SearchValue_0=EJ126258&ERICExtSearch_SearchType_0=no&accno=EJ126258