A tzuras hapesach is a halachic partition considered a full enclosure in every respect. The reasoning behind this form of closure is that in every enclosed area there are openings that allow entry and exit. There is no limit to how many openings may be made in an enclosed structure; some houses have one opening, and some have many openings.
The structure of a tzuras hapesach consists of three parts, each with its own halachos:
The vast majority of contemporary eruvin are built from tzuras hapesach. The poles serve as the doorposts, and the eruv wire serves as the lintel. It is sufficient to learn the halachos of tzuras hapesach in order to inspect the poles and wires of the various eruvin and evaluate their validity
It is appropriate to mention the view of the Rambam, who holds that one cannot enclose an entire area solely with tzuras hapesach; rather, at least 50% of the perimeter must consist of actual partitions, and only the remainder may be closed with tzuras hapesach. His reasoning may be that we do not find a house made entirely of openings, and only when a significant portion is solid may one add many openings. The Shulchan Aruch cites the Rambam’s view as a yeish omrim, and the Mishnah Berurah writes that one should be stringent like his opinion.
In large eruvin it is not feasible to be stringent like the Rambam, and almost all eruvin are lenient against his view, except in neighborhood eruvin and in communities or bases enclosed by a fence.
