Cash-settled bitcoin futures is now a two-horse race as the CME Group began trading its version of the digital currency contract on Sunday night.

During the first three hours of trading, the new contract witnessed $50 million in transactions, which is $20 million more than trade on rival Cboe Global Markets’ CBOE Futures Exchange during the same period, reported the Financial Times.

Although each exchange operator offers futures contracts on bitcoin, there are significant differences between the size of the contracts each offer as well as how and when the exchanges determine their respective reference rates for their contracts’ underlying cryptocurrency.

The newly launched CME Group’s BTC contract represents the purchase of five bitcoins while the Cboe’s XBT contract is written for one bitcoin.

Regarding their reference rates, the CME Group uses the CME CF Bitcoin Reference Rate, which it developed in conjunction with Crypto Facilities using IOSCO Principles for Financial Benchmarks. The BRR represents the price of one bitcoin at 4 pm London time based on data provided by Bitstamp, GDAX, itBit, and Kraken bitcoin exchanges.

The Cboe, on the other hand, has entered into a relationship with Gemini Trust Company, a digital exchange with a bitcoin license from the New York State Department of Financial Services, to provide bitcoin pricing.

The CME sets its BBR daily at 4 pm London time while CFE takes in a real-time feed from Gemini.

Earlier this month, a Nasdaq client leaked the exchange operator’s plan to launch its offering sometime in the first half of 2018, a source familiar with the situation told Markets Media at the time.

Nasdaq tentatively expects to use a bitcoin index it developed itself, which sources data from approximately 50 exchanges and updates every second, added the source.

The Intercontinental Exchange is the only major derivatives exchange operator that has not announced plans regarding digital currency futures and declined to comment for this story.

However, NYSE was one of the first movers into digital currencies market when it made a $75 million investment in the Coinbase exchange in January 2015 and launched its NYSE Bitcoin Index four months later.