Multiple barriers also make it harder for Ug99 to overcome them all.

ECONOMIST: Wheat rust and world farming

Those plans include robust, multiple barriers between pressure regimes and the surface.

FORBES: Deepwater Ban Lifted; Big Oil Lesson Learned

But where multiple barriers exist, progress has been glacial.

ECONOMIST: Women and jobs

Another common reason that customers use multiple brands is structural barriers that distort demand.

FORBES: The Wallet Allocation Rule and Your Brand

Afghanistan's only five-star hotel is also a virtual fortress, surrounded by blast barriers, metal detectors, multiple reinforced gates and heavily armed guards.

ECONOMIST: A bombing in Kabul