The core mechanism underlying every financial market — equities, futures, foreign exchange, commodities — in which participants who wish to buy submit bids (the maximum price they are willing to pay) and participants who wish to sell submit offers, also called asks (the minimum price they are willing to accept). Trades occur only when the two sides meet. The gap between the highest outstanding bid and the lowest outstanding offer is the bid–ask spread, Spread = P_ask - P_bid, which in liquid futures often equals a single tick.