Depending on where you sit, the U.S.-Mexico border is:
a) a dangerous frontier that allows drug traffickers and illegal immigrants to cross freely into the U.S.
b) a familiar frontier that is navigated as a regular part of everyday life.
For people who live along the border in the twinned cities of Nogales, Ariz., and Nogales, Mexico, it's nearly always the latter.
The sister cities are known collectively as Ambos Nogales, or "Both Nogales."