Keystone markers are a now-defunct system of roadside signage developed by Pennsylvania Department of Highways shortly after the First World War, placed outside cities, towns, and boroughs in the state of Pennsylvania.[1] Variations of the marker could be found at highway crossings of creeks, rivers, trails, borough lines, and other points of interest.[2]
^Graham, John (June 2010). "A Welcoming Sign"(PDF). Pennsylvania Borough News. Archived from the original(PDF) on 2014-08-26.
^Graham, John T (2010). "History in Cast Iron"(PDF). Pennsylvania Recreation and Parks Magazine (Second Quarter 2010). Archived from the original(PDF) on 2014-08-26.