Stocks and bonds. The basic building blocks from which most investors construct a well-balanced portfolio. Yet while most individual investors have a reasonable understanding of stocks as shares of ownership in a public company, they are often less familiar with bonds and how they work.
The U.S. bond market is massive: $51 trillion, compared with a total $80 trillion in market value for all U.S. stocks. However, while there are about 12,000 individual stocks, the bond market includes over 1.5 million distinct individual issues, according to the Securities Industry and Financial Markets Association.
Yet despite the nearly endless variety of fixed-income choices and their essential utility in building diversified portfolios, many investors express uncertainty about how they work and may even avoid holding bonds due to their lack of understanding. Today we look at the basics of bonds about how they operate. Next week we will discuss the investment case for holding them.
While stocks represent ownership, bonds are simply loans chopped into pieces and sold to investors. Holders are therefore creditors who have loaned money to a company or governmental entity in exchange for a stream of periodic interest income, called the “coupon” payment, with the promise of full repayment of the principal, called the “par” value at the end of the term. Within that construct there are dozens of variations, but the basic concept applies to all flavors.
The word “bond” traces to the 12th-century Middle English “band,” something that binds or ties together. Yet the concept is far older, reaching back to around 2400 BCE. A clay tablet discovered in Mesopotamia (modern-day Iraq) is believed to be the oldest extant example, a contract that guaranteed payment of a debt in corn, a common currency of the day.
During the 1100s, the city states of Venice began issuing perpetual debt obligations, known as “presiti,” to finance wars with rival states. Over the next two centuries, these obligations were frequently traded when the holder required liquidity, establishing the first early bond market.
In 1693, the Bank of England issued the first national or “sovereign” bonds to finance Britain’s war with France. It was also the pressing need for cash to support the American Revolution that led the Continental Congress to issue short-term bills in 1775. The U.S. Treasury funded World War I with “liberty bonds” in 1917, increasing the national debt to a staggering $5 billion. Today, a total of $31 trillion in treasury debt is outstanding.
Smaller companies in the private sector often relied on bank loans to finance their operations and expansion plans. But beyond a certain level, large corporations like Union Pacific, John Deere or DuPont could not raise enough debt capital from banks and turned to public investors, following a pattern established by the Dutch East India Company. Today, the American corporate bond market totals $12 trillion, second in size only to the U.S government debt.
New bonds are issued in what is called the primary market; that is, the issuer receives the proceeds from the sale, and the buyer becomes a lender. However, most investors acquire them in the secondary market from another holder who wishes to cash out. They are effectively “used” bonds that change hands in the marketplace, analogous to how stocks trade. While the original issue price is typically at or close to the par or maturity value, bond prices in the secondary market fluctuate based upon variation in the economic landscape.
Perhaps the largest single factor that influences bond prices is interest rates or, more specifically, changes in the level of interest rates. The price of an outstanding bond moves inversely with the direction of rate changes in the broader economy. For example, when the current market rate on a 10-year U.S. Treasury bond rises, the value of a similar outstanding bond falls. Many investors have witnessed this phenomenon to their dismay, as rates have climbed significantly in 2026. Conversely, declining rates render the interest income on outstanding bonds more valuable and therefore bond prices tend to rise.
The magnitude of the price movement with respect to rate changes depends most directly on two factors: the time to maturity of the bond and the coupon rate that was fixed when the bond was issued. Bonds with longer maturities and lower coupon rates typically experience larger price changes when rates move.
The interplay between maturity and coupon rates is captured in an important concept called “duration.” Unlike in common parlance, duration is not the same thing as the time to maturity but can be thought of as a modification of maturity that considers the periodic income received over the life of the bond. Think of it as the average amount of time it will take to recoup your initial investment when interest income is factored in. Since the investor receives some of their return with each payment, duration is shorter than maturity.
Although the math is beyond our scope, investors can put duration to work when evaluating bonds or bond funds. Duration also measures the approximate percentage change in the price of a bond given a 1 percentage point change in interest rates, a metric that can be particularly useful in a climate of rate volatility. For example, a 10-year treasury bond with a coupon interest rate of 5% has an effective duration of around eight years, since you get part of your original investment back with each interest payment.
Now suppose interest rates rise suddenly from 5% to 6%. A $1,000 par bond with a duration of eight years would decline in value by 8%, or about $80 in current market value. This is extremely valuable in evaluating the attractiveness of a particular bond or fund and in selecting bonds with less interest rate sensitivity if you believe rates are likely to increase.
Other factors also come into play including credit quality of the issuer, but understanding how prices move with changes in market interest rates is a key step in gaining comfort with bond investing.
Christopher A. Hopkins, CFA, is a co-founder of Apogee Wealth Partners in Chattanooga.