Eruv עירוב

On the Sabbath one may not carry from a private domain into a public one. But a courtyard of several families, a street, a whole quarter—whose are they? The sages gave a way: the residents contribute food, symbolically becoming one household, and the district is enclosed by an unbroken boundary—here a wall, there a wire strung on poles. Within it, carrying is permitted.

The Talmud says Solomon instituted eruvin—and a heavenly voice approved. Today an eruv surrounds almost every Jewish neighbourhood in the world, and most passers-by never know they walk beneath its wire; every Friday an inspector walks the line to check it is whole. The laws fill the tractate of that name—the most diagram-ridden in the Talmud.