Back, lay, match
How an Exchange Takes a Bet
You are looking for a person who wants the opposite outcome. If they take your price, you are matched. If they don’t, you wait. That is the whole model.
The two sides
Back
You think it happens. You take a price if someone will lay it.
ExampleBack City to win at 2.50
Lay
You think it does not happen. You offer the odds. The backer’s win is your liability.
ExampleLay City to win at 2.60
How the money moves
Ten pounds back at 2.50: if City win, the layer owes you £15 profit before commission. If City don’t, you lose the ten. Flip the ticket and you are the layer — same market, inverted liability.
In-play you can work the other side: back at one number, lay at another, and lock a profit or cut a hole before the result is in. Those 2.50 / 2.60 figures are teaching numbers, not a live board. In-play and trading out is the longer walk. The worked arithmetic — including the $0 / $1 ticket — is in what a lay can cost you.
Same idea, different ticket (US readers)
Classic UK and European exchanges use a back-and-lay ladder: you bet an outcome will or will not happen, and another customer takes the other side.
US exchange-style sports venues such as ProphetX and Novig usually trade event contracts (often priced toward $0 / $1) on a CFTC-regulated book. Matching is still matching. The ticket is a contract, the age gate is often 21, and the map is a location check — not a UK residence list. Read those reviews as their own product. They are not Betfair with a US login. Event contracts versus back and lay · where you live versus where you stand.
More on the language, if you want it: Event contracts versus back and lay · Back and lay · Commission vs sportsbook margin · Liquidity and the empty book · How to read a fee table · Lay liability and contract PnL · UK/EU residence vs US location · In-play and trading out · Closed desks and stale rankings · Exchanges we cover