The narrow neck of land was a small isthmus between the land northward and the land southward, lying on the line between the land Bountiful and the land Desolation. It was a day and a half’s journey for a Nephite from the east to the west sea, and the land of Nephi and the land of Zarahemla were nearly surrounded by water on either side of it (Alma 22:32).
Hagoth, described as a curious man, built a large ship on the borders of Bountiful, by the land Desolation, and launched it into the west sea by the narrow neck that led into the land northward (Alma 63:5). The Jaredites earlier built a great city by the narrow neck of land, at the place where the sea divides the land (Ether 10:20).
The neck functioned as a military chokepoint controlling passage between the two lands. Moroni’s armies headed Morianton’s fleeing people at the narrow pass that led by the sea into the land northward (Alma 50:34). Captain Moroni sent orders to fortify Bountiful and secure the narrow pass into the land northward so the Lamanites could not seize it and harass the Nephites on every side (Alma 52:9). The Nephites fortified a line from the west sea to the east to defend their north country, a line that was a day’s journey for a Nephite (Helaman 4:7). In Mormon’s day the same passage marked the boundary of a treaty: the Lamanites ceded the land northward to the Nephites as far as the narrow passage that led into the land southward, and the Nephites gave the Lamanites all the land southward (Mormon 2:29).
The record does not give the neck’s exact dimensions or location.