What Is a Bond? Understanding Fixed Income Securities

A bond is a loan. When you buy one, you are lending money to a government or a company; they pay you interest on a schedule and return your principal at maturity. So far, no different from lending money to a friend.

The difference comes one step later: this loan can be resold. You don’t have to wait thirty years for your principal back — you can sell the bond to someone else at any time. Once a loan is tradable it has a market price, and once it has a price, you get the thing this article is really about: the seesaw between bond prices and interest rates. Rates go up, bond prices go down. Rates go down, bond prices go up. Understand that seesaw and most of what happens in bond markets stops being mysterious.

The market is worth your time. Global bond markets are larger than stock markets (~$100+ trillion), they are how countries and companies finance themselves, and bond yields supply the discount rate used to value stocks. Even if you only ever trade equities, the bond market is setting parameters inside your valuation model.

A Standardized Loan

A bond compresses the loan terms into a few numbers. The face value (or par value) is what the borrower repays at maturity — usually $1,000 for corporate bonds. The **coupon rate** sets the interest: a 5% coupon on $1,000 face pays you $50 a year. The maturity date is when the principal comes back, anywhere from one year to thirty-plus.

The fourth number isn’t printed on the bond; the market computes it. The yield to maturity (YTM) is the annualized total return you actually earn if you buy at today’s market price, hold to maturity, and every payment arrives as promised. The coupon rate is fixed at issue; the yield to maturity moves with the market price. The gap between those two numbers is the hinge of the seesaw.

Pricing: Discounting the Future Back to Today

A bond’s price is the present value of all its future cash flows — each coupon plus the face value at maturity — discounted at the yield to maturity:

where $C$ is the annual coupon payment, $F$ the face value, $y$ the yield to maturity, and $T$ the years to maturity.

Look at the structure: $y$ sits entirely in the denominators. When $y$ rises, every term shrinks and the price must fall; when $y$ falls, the price must rise. The seesaw isn’t market psychology — it’s arithmetic.

Check it with a $1,000 face, 5% coupon, 3-year bond. When the YTM is also exactly 5%, the price is exactly $1,000 (at par) — the discount rate and the coupon cancel. If market rates rise to 6%, newly issued bonds pay 6%, and nobody will take your 5% bond off your hands at the old price; it falls below $1,000 (a discount), precisely far enough that the buyer earns 6% on the deal. If rates fall to 4% instead, your 5% coupon is suddenly generous, and the price rises above $1,000 (a premium).

Duration: The Length of the Lever Arm

The direction of the seesaw is settled. The next question is magnitude: rates move 1% — how much does the price move? The answer differs by bond, and the tool that measures it is duration.

Modified duration reads directly: it approximates the percentage price change for a 1% (100 basis point) move in yields. A bond with modified duration of 8 falls roughly 8% when yields rise 1%; one with modified duration of 2 falls only about 2% in the same scenario.

You can reason out what drives it. The longer your money is locked into the future, the more a rate change hurts — so longer maturity means higher duration. A higher coupon returns your money faster, so it lowers duration. And a higher yield shrinks the discounted weight of the distant cash flows, which lowers duration too. This is why long-term government bonds are so sensitive to rate moves: long maturity, high duration, and the biggest losses when rates jump. What happened to long-bond holders in the 2022 hiking cycle is duration made visible.

The Yield Curve

Plot the yields of same-credit-quality bonds against their maturities and you get the yield curve — the market’s entire set of expectations about future rates and growth, drawn on one chart.

In its normal state the curve slopes upward: long yields exceed short yields, both because locking money up longer demands compensation and because the market expects normal growth. The curve can also invert — short yields above long yields — and historically that has often preceded recessions, which is why every inversion makes headlines. A flat curve, in between, says the market can’t make up its mind. This is why the yield curve is one of the most watched indicators in finance.

US Treasuries provide the curve’s anchor points across maturities: Bills (up to 1 year), Notes (2–10 years), and Bonds (20–30 years). Other major markets have their equivalents — UK Gilts, German Bunds, Japanese JGBs.

The Other Axis: Credit Risk

Everything so far assumed the borrower repays. The second axis asks: will they?

Government bonds are usually treated as the zero point on this axis — lowest risk. But lowest is not zero: Greece in 2012 and Russia in 1998 both reminded the market that sovereigns default too. Corporate credit is priced explicitly along a scale. Investment-grade corporate bonds come from large, stable companies, rated BBB- or above by S&P. High-yield bonds — the market bluntly calls them junk — are rated BB or below, carry higher default risk, and must offer higher yields before anyone will buy them. Two other common flavors: municipal bonds, issued by local governments and often tax-exempt in the US, and TIPS (Treasury Inflation-Protected Securities), whose principal adjusts with inflation — the US government’s inflation hedge in bond form.

The rating agencies (S&P, Moody’s, Fitch) grade the will-they-repay question:

Ratings map to yields by the same logic as the rate seesaw: the higher the rating, the smaller the risk premium investors demand, the lower the yield; the lower the rating, the higher the yield has to be to compensate for default risk. A bond’s yield is, at bottom, the risk-free rate plus this credit premium — both axes meet in a single number.

The Quant’s View

For quantitative work, bonds are not an optional asset class; they are infrastructure. The risk-free yield curve is the foundation under the discount rate used to value every asset — the discount rate in your equity model comes from here. Duration and credit-spread exposures are systematic risk factors in their own right, and a portfolio’s rate sensitivity is described through them. In many market regimes bonds and stocks are negatively correlated — bonds rally when stocks sell off — which makes bonds the most common diversifier in portfolios, though that correlation does not hold in every regime. And bond volatility, especially at the long-duration end, is a key input to portfolio risk models; a risk model that ignores it will be badly wrong in years when rates move hard.

Compressed to one sentence: a bond is a tradable loan, its price and yield sit on opposite ends of a seesaw, duration is the length of the lever arm, credit ratings price the default risk, and the yield curve draws the market’s expectations on a single chart. For the textbook-depth version, Fabozzi’s two books are the standard fixed-income references, and Bodie, Kane, and Marcus’s Investments (Ch. 10–11) places bonds back inside the full investment framework.

References

• Fabozzi, Fixed Income Securities. Wiley, 2011. ISBN: 978-0-470-39970-1.
• Fabozzi, Bond Markets, Analysis and Strategies. Pearson, 2016. ISBN: 978-0-13-388187-6.
• Bodie, Kane, Marcus, Investments, Ch. 10-11. McGraw-Hill, 2020. ISBN: 978-1-26-057101-6.