Bountiful controls the narrow neck of land. This passage locates Bountiful’s northern border against the land Desolation, which the Jaredites had inhabited.
When the Nephites fortify their land “against the Lamanites, from the west sea, even unto the east” they are apparently focusing their fortifications across the narrowest part of the land. Since they have forfeited Zarahemla, they are making their stand at the last defensive line, a line the Lamanites have not breached in the past.
Verse 7 describes the width of the narrow neck as “a day’s journey for a Nephite.” Alma 22:32 had earlier described the time-distance traveling across the dividing line between Bountiful and Desolation as: “And now, it was only the distance of a day and a half’s journey for a Nephite, on the line Bountiful and the land Desolation, from the east to the west sea; and thus the land of Nephi and the land of Zarahemla were nearly surrounded by water, there being a small neck of land between the land northward and the land southward.” Because this passage mentions the “small neck of land,” it appears that we have two different distances for perhaps the same geographic feature.
The difference in the two time-distances is significant: a day’s journey versus a day and a half. Although a sea-to-sea measurement is an easy inference from statements, neither statement is explicit on that point. I argue that the difference stems from measuring different sections of the isthmus. In the Helaman statement, the measurement is “on the line which they had fortified and stationed their armies to defend their north country.” This smaller distance may be measuring to the area later called the land of Joshua (Morm. 2:6).