Bond Basics

A bond is a debt security: the issuer borrows money from the investor, promising to pay periodic interest (coupon) and return the original amount (principal) at a fixed date (maturity). The three pillars—principal, coupon, maturity—determine the bond’s characteristics, risk, and return.

Principal: the foundation of the bond

The principal (or “face value,” “par value,” “notional”) is the amount borrowed. If you buy a bond with $1,000 principal, you are lending $1,000. The issuer promises to return exactly $1,000 on the maturity date. Principal risk exists only if the issuer defaults (fails to repay). Absent default, principal is guaranteed—but principal can lose purchasing power if inflation erodes it.

Bonds are typically issued in units of $1,000 (institutional bonds) or $5,000 (some corporate and municipal bonds). Retail investors often buy bond funds or ETFs to gain access to diversified bond holdings, since purchasing individual bonds in small quantities is inefficient. The total return on a bond comes from two sources: coupon income (the promised interest) and principal gain or loss (if the bond’s market price changes before maturity).

Coupon: the income stream

The coupon is the periodic interest payment, typically paid semi-annually (twice per year) in the U.S. market. A bond with a 5% coupon and $1,000 principal pays $50 per year (or $25 semi-annually). This is straightforward, but the key is that the coupon amount is fixed for most bonds. Even if market interest rates rise to 7%, a 5% bond still pays only $50 per year—the coupon rate does not adjust.

This creates a price-coupon relationship. If prevailing rates rise above the coupon rate, the bond’s market price falls below par (it is now less attractive than newly issued bonds paying higher rates). If prevailing rates fall below the coupon rate, the bond’s market price rises above par (it is now more attractive than newly issued bonds paying lower rates). This inverse relationship between market rates and bond prices is fundamental to bond trading and duration risk.

Some bonds have floating-rate coupons that adjust with a market benchmark (LIBOR, Treasury rates, inflation); these are less sensitive to rate changes. Zero-coupon bonds pay no periodic coupon; instead, they are issued at a deep discount and redeemed at par, with the gain implicitly serving as interest.

Maturity: the time horizon

The maturity is the date when the issuer repays the principal. U.S. Treasury bills mature in days to months. Treasury notes mature in 2–10 years. Treasury bonds mature in 20+ years. Corporate bonds typically mature in 5–30 years. Longer maturity means more interest-rate risk: if rates rise after a bond is purchased, a 30-year bond’s price will fall much further than a 5-year bond’s, because the higher-rate bonds are competitive for longer.

Duration quantifies this maturity-related risk. A bond’s duration (measured in years) tells you roughly how much the price changes for a 1% move in yield. A bond with 10-year duration loses about 10% in value if yields rise 1%; a bond with 5-year duration loses about 5%. Duration depends on maturity and coupon—longer bonds have longer durations, lower-coupon bonds have longer durations (because more of the return comes from principal repayment far in the future, which is riskier to interest-rate moves).

Par, premium, discount: the bond price puzzle

A bond’s market price fluctuates based on prevailing interest rates and credit risk. If you buy a 5% bond when prevailing rates are 5%, you pay par ($1,000). If rates rise to 7%, new bonds pay 7%, so your 5% bond trades at a discount (below $1,000) to equalize the yield. If rates fall to 3%, your 5% bond trades at a premium (above $1,000) because it is now attractive.

Here’s the critical insight: if you hold the bond to maturity, you get back exactly $1,000 principal, regardless of what you paid. If you paid $950 (discount), you earn a capital gain of $50. If you paid $1,050 (premium), you suffer a capital loss of $50. Over time, as the bond approaches maturity, its price converges to par (this is called “pull to par”). The convergence is certain—absent default—but the path is volatile if rates are volatile.

Yield: the comprehensive return measure

The yield to maturity (YTM) is the internal rate of return (IRR) of a bond purchased at the current market price, held to maturity, and cashed at par. It incorporates coupon payments, price appreciation/depreciation, and maturity, all as a single annualized percentage. A bond trading at a discount has a YTM higher than its coupon rate (the YTM captures both coupon income and the gain from price appreciation to par). A bond trading at a premium has a YTM lower than its coupon rate (coupon income is partially offset by the loss from price depreciation to par).

The current yield is simpler: annual coupon divided by current market price. It ignores price convergence to par. A bond bought at $950 with a 5% coupon has a current yield of 5.3% ($50 / $950) but a YTM higher still (because of the $50 gain to par).

Bond types and structural features

Bonds come in varieties that modulate these fundamentals. Callable bonds give the issuer the right to repay early if rates fall—this limits your upside if rates drop (the bond is called away at par) but you retain the downside if rates rise. Putable bonds give the investor the right to sell back to the issuer at par if rates rise—this limits downside. Convertible bonds can be converted into equity, adding an option component.

Treasury bonds are backed by the full faith and credit of the U.S. government (negligible default risk). Corporate bonds are issued by companies and carry credit risk—the company might default. Municipal bonds are issued by state and local governments and are often tax-exempt. Each type has a different risk-return profile and tax treatment.

The essential toolkit

Once you understand principal, coupon, maturity, and their interactions through price and yield, you have the essentials of bond analysis. More advanced concepts—duration, convexity, credit spreads, ladder strategies—build on this foundation. Beginners often buy bonds and hold them to maturity, collecting coupons; intermediate investors actively trade bonds to capture price moves as rates change; advanced investors engineer complex bond portfolios to optimize after-tax returns and hedge against inflation or interest-rate shocks.