The Transportation Security Administration (TSA) has on its website a diagram of its “defense-in breadth” strategy. I got a kick out it because you could liken it to the “defense-in-depth” strategy of many network security professionals.
The TSA says “each one of these layers alone is capable of stopping a terrorist attack. In combination their security value is multiplied, creating a much stronger, formidable system.” So if you get through one layer, the argument goes you’ll get caught in another layer. Maybe 21 layers is the secret number for network security as well? Just a thought.