It is forbidden on Shabbat to carry something four Amot or more in a public domain – meaning, to pick something up, walk four Amot or more, and then stop or put the object down. This prohibition has been transmitted as a "Halacha Le’Moshe Mi’Sinai" – an oral tradition dating back to Sinai.
The length of four Amot is generally assumed to equal approximately 1.9 meters, or six feet, three inches. However, the Shulhan Aruch (Orah Haim 349) brings an opinion that with respect to this prohibition, the measurement of four "Amot" is not actually four Amot, but rather the length of the hypotenuse when two four-Amot lines are placed at a right angle. In other words, if we can imagine a square measuring 4x4 Amot, the length of four Amot with respect to this prohibition is, according to this opinion, the length of the diagonal line extending from one top corner to the opposite bottom corner. This comes out to be 5 and 3/5ths Amot, which equals approximately eight feet, nine inches. After stating this position, the Shulhan Aruch then brings a different view – which he introduces with the expression, "Ve’yesh Omerim" ("And there are those who say") – maintaining that one violates this prohibition by carrying the actual distance of four Amot. The rule "Stam Va’yesh Halacha Ke’yesh" establishes that when the Shulhan Aruch brings two opinions in this fashion, the first is accepted as Halacha. In light of this principle, we accept the first opinion cited by the Shulhan Aruch, that one does not violate the prohibition against carrying through a public domain on Shabbat unless he carries a distance of eight feet, nine inches.
As far as Torah law is concerned, one may circumvent this prohibition by carrying something less than this distance, stopping, and then walking again less than this distance, and so on, until he reaches his destination. However, the Sages enacted that this may be done only by multiple people, but not by one person. For example, if a young child refuses to walk on his own in a public domain, one may carry the child a distance of less than eight feet, nine inches, and then hand the child to somebody else, who would then walk less than this distance and then hand the child to a third person, and so on. The Bi’ur Halacha (Rav Yisrael Meir Kagan of Radin 1839-1933) raises the question of whether two people may carry something in this fashion by handing the object back and forth to one another. Although there are those who permitted carrying in the fashion, the Bi’ur Halacha is inclined to rule stringently, and to permit each person to carry only once.
It must be emphasized that although an item may be carried in this fashion through a public domain, with one person carrying less than eight feet, nine inches and then handing it to somebody else, the item may not then be carried into a private domain. Carrying from one domain to another domain on Shabbat is forbidden regardless of distance. Even if the object is located right outside the home, it is forbidden to bring it inside from the public domain.
In conclusion, we should note that under certain extenuating circumstances, Halacha allows even one person to repeatedly carry an object less than four Amot, stop, and then continue. Namely, if a person was walking with his wallet on Friday afternoon and Shabbat began, he is allowed to carry his wallet through a public domain in this fashion – walking less than four Amot, stopping, and then walking again – until he reaches a place where he can hide it safely.
Summary: The Torah forbids carrying an object a distance of four Amot – eight feet, nine inches – in a public domain on Shabbat. One may carry an object less than this distance through a public domain, but only once. He may not carry less than this distance, stop, and then continue walking, though he may then give it to somebody else to carry it less than four Amot, and that person may then give it to a third person, and so on. An exception is made in the case of somebody who was walking with his wallet in a public domain when Shabbat began, who may repeatedly walk less than four Amot and stop, until he reaches a place where he can safely store his wallet. It must be noted, however, that one may not bring an object on Shabbat from a public domain into a private domain, or vice versa, no matter how small a distance is involved. It is only within the public domain that we allow one to carry less than four Amot and then hand the object to somebody else.