The West begins where the average annual rainfall drops below twenty inches. When you reach the line which marks that drop—for convenience, the one hundredth meridian—you have reached the West.

De Voto defines “the West” not as a political boundary or romantic idea but as an ecological threshold. The twenty-inch rainfall line marks the shift from humid, rain-fed agriculture to a drier regime where settlement, farming, and economic life depend on irrigation, water rights, and different patterns of land use. By tying the West to the 100th meridian “for convenience,” he also signals that the boundary is approximate—useful for thinking, not a hard line on the ground. The remark underscores a central theme in western history: environment and water scarcity shape institutions, conflicts, and culture as much as (or more than) frontier mythology.