yield and price of a bond are inversely related so that when market interest rates rise, bond prices fall and vice versa. For a discussion of the mathematics In finance, a bond is a type of security under which the issuer (debtor) owes the holder (creditor) a debt, and is obliged – depending on the terms – to provide cash flow to the creditor; which usually consists of repaying the principal (the amount borrowed) of the bond at the maturity date, as well as interest (called the coupon) over a specified amount of time. The timing and the amount of cash The market price of a bond is the present value of all expected future interest and principal payments of the bond, here discounted at the bond's yield to maturity (i.e. rate of return). That relationship is the definition of the redemption yield on the bond, which is likely to be close to the current market interest rate for other bonds with similar characteristics, as otherwise there would be arbitrage opportunities. The yield and price of a bond are inversely related so that when market interest rates rise, bond prices fall and vice versa. For a discussion of the mathematics see Bond valuation. The bond's market price is usually expressed as a percentage of nominal value: 100% of face value, "at par", corresponds to a price of 100; prices can be above par (bond is priced at greater than 100), which is called trading at a premium, or below par (bond is priced at less than 100), which is called trading at a discount. The market price of a bond may be quoted including the accrued interest since the last coupon date. (Some bond markets include accrued interest in the trading price and others add it on separately when settlement is made.) The price including accrued interest is known as the "full" or "dirty price". (See also Accrual bond.) The price excluding accrued interest is known as the "flat" or "clean price". Most government bonds are denominated in units of $1000 in the United States, or in units of £100 in the United Kingdom. Hence, a deep discount US bond, selling at a price of 75.26, indicates a selling price of $752.60 per bond sold. (Often, in the US, bond prices are quoted in points and thirty-seconds of a point, rather than in decimal form.) Some short-term bonds, such as the U.S. Treasury bill, are always issued at a discount, and pay par amount at maturity rather than paying coupons. This is called a discount bond. Although bonds are not necessarily issued at par (100% of face value, corresponding to a price of 100), their prices will move towards par as they approach maturity (if the market expects the maturity payment to be made in full and on time) as this is the price the issuer will pay to redeem the bond. This is referred to as "pull to…