688. The Diagonal of a Four-Cubit Square

Shabbos 12:17

Based on principles detailed in the previous few halachos, it is permitted for a person to pick an object up in the public domain and give it to a friend who is within his four-cubit box. The friend may then give it to another friend who is next to him, and so on. This is permitted even if the object changes hands hundreds of times, even if it is transported several miles on Shabbos, because each person only moved it within his permitted four cubits.

Shabbos 12:18

Since a person may carry within a square that is four cubits by four cubits, he may carry along the diagonal of this square, which is rounded down to 5 3/5 cubits long (about 8.5 feet). Therefore, if a person carries or throws an object in the public domain, he is not liable unless he transferred it more than 5 3/5 cubits. Accordingly, when we speak of moving an object four cubits in the public domain, we are referring to the four-cubit by four-cubit box whose diagonal is 5 3/5 cubits. One would not be liable for transporting a distance less than this.