In a fragmented global market, price quotes are all over the place. That's why CoinDesk created the Bitcoin Price Index (BPI). The current debate over what exactly is the all-time-high price of the leading cryptocurrency shows how a sector that is all about decentralization has difficulty coming up with a common pricing system on which everyone can agree.

Depending on whom one asks, the price of bitcoin at any given time can be quoted within a range of hundreds of dollars. For that reason, some outlets are claiming bitcoin has already eclipsed its all-time high set three years ago, while others, including CoinDesk, are saying we're still a ways from the mark. That’s because much like the foreign exchange market, the cryptocurrency market isn’t centralized the way, say, the New York Stock Exchange is.

While individual crypto exchanges are called “centralized,” they are to the extent that trades take place within their platforms. In fact, they are trading the same digital assets found on other exchanges.

One could send bitcoin bought on one exchange to another and then sell it on that second exchange. That opens up arbitrage potential that helps keep prices relatively close together, though with sizable discrepancies. Slippage, differences in liquidity and other costs help explain why prices may be so different across the board.

Likewise, the historic all-time high for bitcoin is also different, depending on the source. CoinDesk’s Bitcoin Price Index uses a weighted average of prices to get a reasonable sense of where bitcoin is or was trading at any particular moment. The average is taken from 10 leading cryptocurrency exchanges including Bitflyer, Bitstamp, Coinbase and Kraken. Were one to get a quote on bitcoin from a major exchange at any time of day, it may not be exactly at the BPI’s number but it is quite likely to be very close. Nonetheless, volatile markets can produce the occasional outliers.

For decades, if not centuries, good old-fashioned fiat currencies have been mostly traded over the counter, first in physical marketplaces, then using telephones and computer terminals. As in crypto, the exact price of a floating-rate fiat currency is a function of supply and demand. Thus when futures exchanges use a reference rate to use for their forex contracts, they first decide on what sources will be used, then plug them into a formula of some kind. In many respects, one can think of CoinDesk's BPI as a sort of reference rate for the price of bitcoin.

And for the record, CoinDesk views the all-time high of bitcoin as $19,783. We'll let you know if and when it gets there.