An eruv allows Jews who adhere to traditonal religious law to "carry " objects in public spaces on the Sabbath, something they are normally prohibitted from doing. "Carrying" in this context includes everything from toting bags to pushing strollers and wheelchairs to wearing knapsacks. The boundary is a real physical entity, albeit invisible to all but the trained eye. It may consist of existing walls, embankments, trees, telephone wires, very slightly modified telephone poles and lampposts, and short lengths of plastic twine. The North Charles Community Eruv is designed to benefit the many traditionally observant Jewish families that live in parts of Cambridge and Somerville. The eruv includes most of Cambridge and roughly half of Somerville.