So far, we've discussed bonds as if investors always buy and hold them until they mature. A lot of people do just that, but many others -- including the pros -- buy and sell them on the open market before they reach maturity. Consequently, the price of a given bond can fluctuate -- sometimes wildly. That means it's unlikely you'll ever be able to sell a bond for "par," or 100% of its face value.

We'll explore what drives price changes in the next lecture, but for now, consider what happens when the price goes up or down. As you already know, a bond's periodic coupon and its ultimate payout never change once the bond is issued. Consider a 30-year bond with a face value of $1,000 and a 6% ($60) coupon. If the price falls to $800, you'll still get $60 each year in interest and $1,000 when the bond matures. The same holds true if the bond's market price jumps to $1,200. Obviously, then, the $800 bond is a much better deal -- you're getting the same payout for $400 less.

OK, So What Does 'Yield' Mean?
Yield -- a bondspeak standard -- is a figure that captures this change in value. It's the percentage return your bond investment promises at any given price.

In its most simple incarnation -- known as "current yield" -- it can be expressed with this formula: Yield = Coupon/Price. When you buy a bond for face value, the yield is simply the coupon, or interest rate. But when the price fluctuates, the yield grows or shrinks to compensate in either direction.

Let's look at that 6% bond again. If you were to buy it for $1,000, the current yield would simply be 6% ($60/$1,000). But if the price drops to $800, the yield rises to 7.5%. Why? Because the guaranteed coupon -- $60 -- is now 7.5% of the $800 you paid for the bond ($60/$800). If the price rises to $1,200, the percentage shifts down to 5%.

Yield to Maturity
Unfortunately, it gets even more complicated. In the real world, when people talk about yield, they're really talking about another figure, called "yield to maturity." This represents the total return you can expect if you buy a bond at a given price and hold it until it matures.

Yield to maturity includes the fact that the bond you bought for $800 will pay you $1,000 when it's due. It also assumes you reinvest the coupons at the same rate and figures in the compounding effect. If, in the above example, you add that $200 difference and the effects of reinvested coupons, the yield to maturity calculates out to 7.73% -- a significantly better deal than the original coupon of 6%.

Once you've grasped the inverse relationship between price and yield, you're ready to take on the bond market's next puzzler: If yields and prices move in opposite directions, how come both high yields and high prices are considered good things?