Debt instruments refer to financial contracts that represent a borrower's obligation to repay a lender. When trading these instruments, the total dollar amount exchanged is determined by multiplying the number of instruments traded on one side of the transaction by their face value. This calculation is crucial in determining the value of a debt instrument and plays a significant role in financial markets. As a knowledgeable professor, I encourage you to delve deeper into the intricacies of debt instruments and their impact on the world of finance.