The spot market

A spot market is a list of people willing to buy (bids) and people willing to sell (asks), each with a size. The best bid and the best ask are the inside of the market. The gap between them is the spread.

“The price” you see on a homepage is usually the last trade, or a mid between bid and ask. It is a description of what just happened, not a guarantee that your next order will print there.

Thin books move a long way on small orders. A coin quoted at a neat number can still cost you more once your order walks up the asks. Size and liquidity belong in the same sentence.

A market order says: fill me now, at whatever is resting on the other side. You get certainty of execution and uncertainty of price. A limit order says: fill me only at this price or better. You get certainty of price and the chance of not trading at all.

On the practice desk, fills use the displayed last price and charge no commission. That is a rehearsal, not a market. Live, you pay the spread, and sometimes you pay slippage when the book is thin or you are late.

Professionals decide the order type before they decide they are excited. If the idea only works at one price, it is a limit. If missing it is worse than paying the spread, it is a market — and the size should be small enough that the book can hold it.