A buy to cover order, or simply buy to cover (BTC) order, is a transaction necessary for traders to close their short positions. It is essentially the opposite of a regular buy order, which is used when traders believe a stock's price will rise and want to acquire additional shares.

When someone takes a short position, they are borrowing money from their broker to enter the trade, hoping that a stock's price will fall. If the price does fall and there is a profit, then the shares are bought back with the profits to close out the trade and return the stock to the original lender. This is where the buy to cover (BTC) order comes in.

A BTC order forces the trader to purchase the stock at the market price regardless of the price changes, or in the most conservative scenario, at a predetermined limit price. The purpose of buying to cover is to pay back the broker, close out the short, and collect the profits that stem from the price fall.

BTC orders are usually entered as a margin trade and incur a margin interest, which is calculated by the trading platform or broker providing the service. The margin interest rate might vary depending on the broker or platform. BTC orders are especially useful when traders enter a long-term short position, as they make sure that their money is returned to their broker and that the position is closed to mitigate losses.

In essence, a BTC order provides a way for traders to efficiently close their short positions on the stock market. By using it, traders can return the stocks to the original lender, avoid extra losses, and collect their profits. However, before entering into any trade, it is important for traders to understand the market and the risks involved in order to trade effectively and with confidence.