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What is a betting exchange?

Consider the following betting websites: William Hill, Betfair, Smarkets, Bet365, Ladbrokes. They appear very similar, but there are two types of company here, and they operate very differently! On one side we have bookmakers, and on the other we have betting exchanges. The traditional bookmakers are William Hill, Bet365, and Ladbrokes. The betting exchanges include Betfair, Smarkets, and Ladbrokes’ exchange. So, what’s the difference?

A traditional bookmaker like William Hill first draws up their own private beliefs in the probability of events, then offers you some fixed odds based on those beliefs. For example, in today’s Liverpool vs. Tottenham game, they offer you:

SelectionDecimal Odds
Tottenham to win6.00
Liverpool to win1.55

You decide on one of these selections to bet on (e.g. Liverpool to win, with odds 1.55), then enter a stake (e.g. $10). The bookmaker accepts, and agrees to pay you $15.50 if Liverpool win.

In contrast, a betting exchange like Betfair allows its users to play the role of bookmaker. Instead of Betfair drawing up its own private beliefs in the probability of events, and then offering you odds based on this, Betfair instead allows its users to offer other users whatever odds they are comfortable with. To do so, Betfair provide a more complex interface like this:

Tottenham to win6.606.80
Liverpool to win1.60 1.61
Draw 4.40 4.50

The values under the “Back” column have a similar meaning to before: if you place $10 on Liverpool to win with odds 1.60, then if Liverpool win, you’ll receive $16; but if Liverpool don’t win, you’ll receive nothing. This is a “back bet”, and it’s similar to what you do at a bookmaker.

The “Lay” column is new. This lets you play the role of bookmaker! If you place a $10 lay bet on Liverpool to win at odds 1.60, you’re effectively allowing another user to place a normal $10 back bet against you. So if Liverpool win, you’ll have to pay them $16; but if Liverpool don’t win, you’ll keep their $10.

Actually, the table of odds that Betfair provide is just a guide. Unlike at a bookmakers, you can place back bets and lay bets at any odds you like, and any stake you like! This then goes into a pool of unmatched bets. For example, Betfair could currently have the following unmatched bets:

In this scenario, could Betfair match Alice and Bob’s bets? No: for Alice’s $10, Bob is only willing to pay her $16 if Liverpool win, but Alice wants at least $16.10. So these bets sit unmatched, waiting for someone to place a new bet which would match with them. Now, imagine that I, Jim, placed a new unmatched bet:

Now, Jim’s bet can be matched with Bob’s! Jim buys $20 of Bob’s $50 offer, leaving Bob with a partially matched bet. This leads us to a new state:

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