Brooklyn and Reshus Harabim
Brooklyn is a continuous borough of open streets joined to the rest of New York by bridges, tunnels, transit, commerce, and daily movement. Rav Moshe's Brooklyn rulings do not treat each neighborhood as a small town with a separate population and street system.
One connected urban network
Neighborhood names describe communities; they do not close the roads between them. Major avenues, buses, subway lines, bridge and tunnel approaches, commercial districts, and continuous development connect Flatbush, Boro Park, Crown Heights, and the rest of the borough.
A proposed eruv may draw a smaller perimeter, but that boundary must be halachically effective. Until it is, the public streets inside remain part of the larger network.
Population and the roads serving it
Rav Moshe's Brooklyn discussion looks beyond the residents inside a proposed route. In Orach Chaim IV:88 he refers to an estimated three million Brooklyn residents and about one million people entering it for work and other needs at that time.
The figures belong to the historical teshuva and should not be presented as current data. Their importance is how Rav Moshe framed the question: the roads serve the population of the connected city, not only the households inside a neighborhood line.
Flatbush and Boro Park
When a report circulated that Rav Moshe had permitted an eruv in Flatbush, he wrote to the Flatbush Vaad HaRabbonim that the report was false (Orach Chaim IV:87). The following teshuva continues the citywide analysis and rejects treating Flatbush as detached from Brooklyn.
Orach Chaim V:28 returns to Brooklyn and Boro Park. It also revisits the concern that public carrying can teach visitors a practice they may later repeat in a place without a comparable enclosure.
The public rulings of 5739
Rav Moshe's signed letter of 17 Teves 5739 states that no eruv should be made in Brooklyn because it is a reshus harabim de'oraisa and rejects reliance on the Boro Park project.
The Kislev proclamation carries sixteen printed names, including Rabbi Zalman Shimon Dworkin, and applies the earlier Manhattan prohibition to New York and other large cities. A further public notice carried seventy-three rabbinic names from a broad range of communities.
Crown Heights
Crown Heights lies inside that Brooklyn street system and has its own direct ruling. Rabbi Avrohom Osdoba and Rabbi Shlomo Segal cite the earlier psak of Rabbi Zalman Shimon Dworkin and the leading poskim headed by Rav Moshe in stating that no valid eruv exists anywhere in Crown Heights.
The Brooklyn sources explain the large-city background. The local Beis Din supplies the practical instruction: the advertised route is not to be used, and carrying remains prohibited.