regulations are discussed in @brito2014bitcoin. Finally, it is clear that without central governance, one cannot impose any form of prohibition on sales of particular items, this point is discussed by @maccarthy2010payment, where they point out that traditional e-money and credit card payment systems regularly monitor and disallow a range of transactions which are deemed unlawful in the place of sale.

Trends in the usage of crypto-currencies in the economy {#sec:trends}
=======================================================

The discussion in the previous section should highlight the much greater potential of crypto-currencies for entering the real economy, compared to virtual currencies. We present in this section summary statistics for the uptake of Bitcoin, the most popular crypto-currency. We also discuss associated investment products, as well as views about the currency’s potential use for facilitating criminal transactions.

Bitcoin trading by exchange and currency
----------------------------------------

Bitcoin is by no means the only crypto-currency. Coinmarketcap[^9] lists 590 currencies, with a total market capitalisation of \$4.5 billion. As Bitcoin accounts for more than 80% of this amount, we will focus on it to exhibit trends in crypto-currency activity. Figure \[fig:volumebyexchangecurrency\] shows the evolution of price, as well as traded volumes over a 2-year period. It is interesting to note that while 