Enter bitcoin. All kind of vibrant economic activity is occurring in this informal economy, which in some regions is between 20-60% of GDP or more, and every economy needs a currency. Essentially, bitcoin is the ‘System D’ of currencies — global, decentralized, and non-state sanctioned. It is still early days but as bitcoin bypasses traditional banking and financial institutions, it is a currency off the grid just as System D. To deny the existence of System D is to deny the fact that economic participants find ways to survive even during prolonged times of hardship. According to Neuwirth “it asserts an important truth: what happens in all the unregistered markets and roadside kiosks of the world is not simply haphazard. It is a product of intelligence, resilience, self-organization and group solidarity.”
http://www.forbes.com/sites/jonmatonis/2012/03/19/could-bitcoin-become-the-currency-of-system-d/