MB4 200a: Carrying from One Domain to Another on Shabbos (346:2-3)

Carrying 4 amot in a public domain is an issur d'oraita. Chazal extended the prohibition to a karmelit.  Carrying from one karmelit to another (less than 4 amot).

Status of a קרפף and a man-made mechitza vs. a natural mechitza.