For the same variety and the same spread, the cost of placing an order may be quite different. The answer lies in the relationship between the order size and the depth of the quotation. This article explains how the thickness of the depth determines the actual transaction price, the impact cost caused by large orders eating up multiple quotations, the logic of choosing between market orders and limit orders, the timing of splitting orders in large transactions, and the law of changes in depth over time, to help traders evaluate the relationship between the order size and the current depth before placing an order.