Order Book

An order book is the running list of buy orders (bids) and sell orders (asks) sitting on an exchange at any moment, ranked by price, that the exchange matches against each other to fill trades.

Each side of the book shows how much someone is willing to trade and at what price: bids stacked below the current market price, asks stacked above it, with the narrow space between the best bid and best ask forming the spread. A "deep" book — lots of size stacked close to the current price — means a trade can fill without moving the price much. A "thin" book means even a modest order can push the price around as it eats through the available levels, which is one of the mechanisms behind slippage. This is also where maker/taker fees come from: an order placed on the book waiting to be filled is a "maker," and one that fills immediately against what's already there is a "taker."

Understanding order books matters even if you never place one, because it explains why the same trade can cost different amounts on different platforms. A pair with a deep, active book tends to have a tight spread and low slippage; an obscure pair with a thin book can have both, even at the same nominal fee rate. It's also why the number you see on an order-book exchange before you click "trade" is only ever an estimate — the book can shift in the seconds it takes you to confirm.

Zest doesn't run an order book. Instead of matching your order against other users' resting orders, a swap here gets a single quoted rate directly from an exchange partner providing liquidity for that pair, with the partner's margin already built into the quote as the spread rather than shown as separate maker or taker charges. You then choose between a fixed-rate order, which locks that quoted rate for a short window, or a floating-rate order, which keeps tracking the market until your deposit is processed — see fixed vs. floating rate for how that trade-off plays out. There's no book depth to check and no risk of your own order moving the visible price, since you're never trading against a public queue of other orders.

Knowing how order books work is still useful background for comparing quotes: it's the reason liquidity, not just a stated fee percentage, determines what a swap actually costs you on any given platform.