Chapter 2, Mishna 3, 4
Upright boards in the corners, i.e. a “virtual enclosure” only works to permit drawing water from public well fed by a spring, per Rabbi Yehuda ben Bava. Such an enclosure does not work for a private well or any kind of cistern that collects water. A enclosure not attached to a dwelling can be up to 5000 square cubits in area – the measure of the courtyard of the Mishkan, Tabernacle, in the wilderness – in order to permit carrying there on Shabbos. (Mishkan actually means dwelling.) It need not contain a structure, dwelling or source of water, and it does not need to be square in shape.