Shulchan Arukh Orach Chayim · Siman 349

English

The Law of Four Amot in the Public Domain, 5 Seifim: Every person has four amot in the public domain in which he may carry. We measure these amot by his own forearm ("amah"). If his forearm is small, we allow him four average amot, which are each six tefachim (and see below chapter 396).

English

These four amot are measured by their diagonal, meaning that he is allowed five and three-fifths amot. There is one who says that from four amot to 5 3/5 amot is exempt but forbidden.

English

A person is permitted to remove an object from the public domain and hand it to his friend who is within his four amot, and that friend to another friend, even though the object will move a few millin, as long as the individual does not remove the object from its techum, and there are those for forbid this (see above chapter 301:42).

English

If there are two individuals such that part of one's amot fall in the other's amot, like if there are six amot in between them, they may bring food to the center and eat as long as neither removes something from his area to his friend's area. If there were three people, with a person in the middle, like if the two outer ones have eight amot in between them, the middle person is permitted to [eat with] the outer ones where their four amot overlap with his, but the two outer ones are forbidden from [eating with] each other.

English

It's forbidden to carry something less than four amot, even at twilight and even in the karmelit.