The border between the United States and Mexico extends 1,951 miles. Neither natural barrier nor physical construct, it has outlived the iron curtains and cement walls of Eastern Europe and has become perhaps the most potent political demarcation of our time. Its permanence can be marked by its permeability. The border does not work to keep people out - in Tijuana there are one million crossings each year and in the desert it may not even keep cattle from wandering.

And yet its power to divide remains undeniable.

There are traces of this division everywhere. These signs - cinderblock churches, rusted generators, undergrazed pastures, the smell of the heat, casual debris - are what William Langewiesche, in the tracker's argot, is "cutting" for - trying to understand. Langewiesche explores this unique terrain, the only place where first and third worlds meet face-to-face, and reveals how the contours of the border affect not only those who live along it but also the rest of us, who live in the border's shadow.