How Bonds Work: Fixed Income, Yield, Duration, and Credit Risk

What Is a Bond? The Basic Structure

A bond is a debt instrument — a loan that an investor makes to a borrower (typically a government or corporation) in exchange for periodic interest payments and the return of the loan amount at a specified future date. When a government or company needs to raise money, it can either sell equity (stocks) or borrow money by issuing bonds. Bond investors become creditors of the issuer; they have a legal claim to the promised interest and principal payments that takes priority over equity holders in the event of financial distress. This priority makes bonds generally less risky than stocks of the same issuer, though they also offer lower expected returns.

The key terms of a bond are straightforward. The "face value" or "par value" is the amount the issuer will repay when the bond matures — typically $1,000 per bond in the United States. The "coupon rate" is the annual interest rate stated on the bond at issuance; a 4% coupon on a $1,000 face value bond pays $40 per year (typically $20 every six months). The "maturity date" is when the issuer repays the face value. The "price" of a bond is what someone actually pays to buy it in the secondary market, which may differ from face value depending on current interest rates and the issuer's creditworthiness. A bond trading at $1,000 is at "par"; above $1,000 is "at a premium"; below $1,000 is "at a discount."

Yield: What You Actually Earn

The coupon rate tells you how much income the bond pays relative to its face value, but the "yield" tells you what you actually earn given what you paid for the bond. The simplest yield measure is "current yield" — annual coupon payment divided by current price. If a bond pays $40 per year and trades at $950, the current yield is 4.21% ($40 / $950). But the more important measure for comparing bonds is "yield to maturity" (YTM) — the total annualized return an investor would earn if they held the bond to maturity and reinvested all coupon payments at the YTM rate. YTM accounts for both the coupon income and the capital gain or loss from buying at a discount or premium to face value.

The relationship between bond prices and yields is inverse and is one of the most important concepts in fixed income: when interest rates rise, existing bond prices fall; when rates fall, prices rise. This makes intuitive sense: if a bond pays 4% and new bonds now pay 5%, your 4% bond is worth less — no one will pay face value for a below-market interest rate. The price must fall enough that the yield (coupon payment / price) rises to match the going rate. Conversely, if rates fall to 3%, your 4% bond becomes valuable — its price rises until the yield falls to the market rate. For investors holding individual bonds to maturity, price fluctuations are less important (you receive face value at maturity regardless); for investors who may need to sell before maturity, price risk is very real.

Duration: Measuring Interest Rate Sensitivity

"Duration" is the key measure of a bond's sensitivity to interest rate changes. Technically, Macaulay duration is the weighted average time to receive all cash flows from a bond, expressed in years. A bond with a duration of 5 years will see its price fall by approximately 5% if interest rates rise by 1 percentage point, and rise by approximately 5% if rates fall by 1 percentage point. "Modified duration" makes this price-sensitivity calculation more precise. The key insight is that longer-maturity bonds and bonds with lower coupon rates have higher duration — they are more sensitive to interest rate changes.

Duration matters enormously for portfolio construction. In an environment of rising interest rates — as occurred in 2022 when central banks hiked rates aggressively to combat inflation — long-duration bonds (such as 20- or 30-year government bonds) suffer large price declines. The iShares 20+ Year Treasury Bond ETF (TLT) fell approximately 33% in 2022 as rates rose sharply, a loss comparable to a stock market correction. Short-duration bonds (under 2-3 years) are much less affected by rate changes and are appropriate for investors who prioritize capital preservation. Investors who need to spend their bond holdings at a predictable time — funding a college tuition payment in three years, for example — should match the duration of their bond holdings to their investment horizon to avoid being forced to sell at a loss.

Types of Bonds: Government, Corporate, and Beyond

The bond market encompasses a vast array of different instruments. US Treasury bonds are issued by the federal government and are considered the world's safest bonds — backed by the full faith and credit of the United States government, which has never defaulted on its obligations. They come in maturities from 4 weeks (Treasury bills) to 30 years (Treasury bonds). Treasury Inflation-Protected Securities (TIPS) adjust their principal value with inflation, protecting investors against purchasing power erosion. Municipal bonds ("munis") are issued by state and local governments; their interest is typically exempt from federal income tax (and often state tax for residents), making them attractive to investors in high tax brackets. "Investment-grade" corporate bonds are issued by financially sound corporations; they pay higher yields than Treasuries to compensate for modestly higher default risk. "High-yield" bonds (also called "junk bonds") are issued by companies with weaker finances; they pay substantially higher yields but carry significant default risk.

Beyond these domestic categories, the global bond market includes bonds from foreign governments (sovereign bonds), emerging market bonds in local and hard currencies, mortgage-backed securities (MBS) and other asset-backed securities (ABS), and various structured products. Each category carries distinct risk and return characteristics. International bonds introduce currency risk — the possibility that currency movements will offset or amplify returns for home-currency investors. Mortgage-backed securities carry "prepayment risk" — when interest rates fall, homeowners refinance, repaying the underlying mortgages earlier than expected and forcing investors to reinvest at lower rates.

Credit Ratings and Default Risk

Not all bond issuers are equally likely to repay their debts. Credit rating agencies — Moody's, Standard & Poor's, and Fitch — analyze the financial condition of bond issuers and assign letter grades reflecting their assessment of default risk. S&P's scale runs from AAA (the highest quality, lowest risk) through AA, A, BBB (still considered investment grade) and then BB, B, CCC, CC, C, and D (default). Moody's uses a similar scale with slightly different notation. Bonds rated below BBB- (S&P) or Baa3 (Moody's) are classified as "high yield" or "speculative grade," and many institutional investors are prohibited by their mandates from holding them.

Credit spreads — the additional yield that corporate bonds pay above equivalent-maturity Treasury bonds — reflect the market's assessment of credit risk. When spreads are narrow, investors are accepting less compensation for taking on default risk, often a sign of economic optimism and investor appetite for risk. When spreads widen sharply — as they did in March 2020 at the onset of the COVID pandemic — markets are pricing in significantly higher default probabilities, and higher-rated bonds tend to outperform. For individual investors, credit diversification — spreading holdings across many different issuers rather than concentrating in a few corporate bonds — is essential to managing default risk, since even investment-grade issuers occasionally default, as the collapse of Enron and Lehman Brothers demonstrated dramatically.

Bonds in a Portfolio: The Role of Fixed Income

Bonds play several important roles in a diversified investment portfolio. Historically, high-quality government bonds have been negatively correlated with stocks during periods of market stress — when stocks fall sharply, investors often "flee to safety" and buy government bonds, pushing bond prices up. This negative correlation provides portfolio-level diversification benefit: a portfolio of 60% stocks and 40% bonds typically experiences lower volatility and smaller drawdowns than an all-stock portfolio, at the cost of somewhat lower expected long-term returns. The traditional "60/40" portfolio remains a widely used benchmark in institutional and retail portfolio management.

The appropriate allocation to bonds depends on an investor's time horizon, risk tolerance, and income needs. Young investors with long time horizons can afford to hold mostly stocks, accepting higher short-term volatility for higher expected long-term returns. Investors approaching or in retirement who rely on their portfolio for living expenses have less ability to wait out bear markets and generally benefit from higher bond allocations. Target-date funds — a popular vehicle in 401(k) plans — automatically shift their allocation from predominantly stocks to more bonds as the target retirement date approaches, implementing this life-cycle principle automatically. Understanding how bonds work is foundational not just for bond investors but for any investor who wants to understand how portfolio risk is managed over time.