# Backing Team A at decimal 2.30 with $100 if A wins: +$130 profit (minus exchange commission) if A loses: -$100 # Identical shape to +130 at a sportsbook; the difference # is who's on the other side (a peer, not the house)
Back prices on a liquid exchange are often better than sportsbook prices on the same outcome because no bookmaker margin is embedded - commission on winnings replaces it. Comparing the two tells you how much vig the fixed-odds book is charging.
Back/lay vocabulary appears whenever exchange prices enter a comparison set. In practice you treat an exchange back price like any other decimal quote: convert to implied probability, adjust for commission, and line it up against the books in the aggregated feed.