Carrying from One Domain to Another
1. It is forbidden, on Shabbos, to carry any article a distance of four
"amos" (1) within a "Reshus HaRabim ("public domain") or a "Carmelis"
(2). [This prohibition applies to] carrying, throwing, or passing the
object. It is also prohibited to carry it in several small stages, [even
though] the distance covered at each interval is less than four "amos" (3).
(1) The "amah" (cubit) referred to here is the distance from the elbow to
the tip of the middle finger. Each person measures this according to his
own body. Also, the maximum distance one may carry is not exactly four
"amos," but the diagonal of a four "amah" square, which is approximately
5.6 "amos"(See Shulchan Aruch 349:1-2 and the Mishna Berura).
(2) See HY 81:3-4 for definitions of "Reshus HaRabim" and "Carmelis."
(3) Carrying an object in small intervals of less than four "amos" is
forbidden only if one will end up carrying a total distance of four "amos."
However, carrying a total distance of less than four "amos" is permissible.