Bonds are the largest asset class in the world by market value — roughly $130 trillion globally — and yet most investors find them deeply unintuitive. The math seems backwards. The terminology sounds technical. Even the relationship between price and yield seems to defy common sense. This article walks through the mechanics from the ground up.
What a bond actually is
At its core, a bond is a loan. When you buy a bond, you are lending money to whoever issued it — a corporation, a government, a municipality — in exchange for two things: periodic interest payments (called coupons) and the return of your original principal when the bond matures.
For example, a 10-year US Treasury bond with a 4% coupon means: you give the US government some amount of money today (say $1,000), they pay you $40 per year (4% of $1,000) every year for ten years, and then at the end of year 10 they return your original $1,000.
That is it. Everything else about bonds — the price moves, the duration math, the yield curve — is just the consequence of these basic mechanics interacting with changing market conditions.
Why prices and yields move opposite
This is the single most counterintuitive thing about bonds, and it trips up nearly everyone the first time they encounter it. The relationship is mechanical — there is no economics involved, just arithmetic.
Imagine you bought that 10-year Treasury at issuance for $1,000 with a 4% coupon. A year later, suppose the Federal Reserve has raised interest rates and newly issued 10-year Treasuries now offer a 5% coupon. Your bond still pays $40 per year. A new bond pays $50 per year. Why would anyone buy your bond at the same $1,000 price when they could buy a new one paying more?
They would not. So if you want to sell your bond, you have to offer it at a discount — say $920 — so that the new buyer effectively earns the same yield they could get on a new bond. Your bond price has fallen. Its yield (the return the new buyer earns) has risen to match the new market rate.
The reverse is true when rates fall. If new bonds are issued at 3%, your 4% bond becomes more attractive, and its price rises so that the yield to a new buyer comes back to roughly 3%.
Price up, yield down. Price down, yield up. The relationship is exact and inviolable. Every bond trade in the world obeys this arithmetic.
Duration: the most important concept
Once you understand that bond prices move inversely to yields, the next question is: by how much? That is what duration measures.
Duration is the percentage change in a bond's price for a 1% change in yield. A bond with a duration of 5 will lose roughly 5% of its price if yields rise by 1 percentage point, and gain roughly 5% if yields fall by 1 point.
Several factors determine duration. Longer-maturity bonds have higher duration than shorter-maturity bonds. Lower-coupon bonds have higher duration than higher-coupon bonds. Both of these properties make intuitive sense: a 30-year bond locks in a payment stream for much longer, so a change in market rates affects more years of payments.
In practice:
A 2-year Treasury has duration around 1.9
A 10-year Treasury has duration around 8.5
A 30-year Treasury has duration around 18-20
This is why long-dated Treasuries are so volatile despite being one of the safest assets in the world. A 1% move in yields — which can happen in weeks — moves the price of a 30-year bond by nearly 20%. That is equity-like volatility.
Yield to maturity vs current yield
There are two common ways to express a bond's yield, and the difference matters.
Current yield is simply the coupon divided by the current market price. If a bond pays a $40 coupon and trades at $920, the current yield is 40 / 920 = 4.35%. This is intuitive but incomplete.
Yield to maturity (YTM) is the more complete measure. It calculates the total return an investor earns if they hold the bond to maturity, including both coupon payments and the difference between the purchase price and the face value returned at maturity. For a bond purchased below face value, YTM is higher than the current yield because the investor gets a capital gain at maturity.
When professionals quote a "yield," they almost always mean YTM. It is the proper apples-to-apples comparison across bonds with different prices, coupons, and maturities.
Credit risk vs interest rate risk
Bonds carry two distinct risks that investors often confuse.
Interest rate risk is the risk that yields rise (and prices fall) before you can sell. This is the risk we have discussed so far — it depends on duration and the path of interest rates.
Credit risk is the risk that the issuer fails to make payments — either misses a coupon or fails to return principal at maturity. US Treasuries are considered to have effectively zero credit risk; investment-grade corporate bonds have low credit risk; high-yield (junk) bonds have meaningful credit risk; and distressed debt is being priced for substantial loss probability.
These two risks behave very differently. Interest rate risk applies uniformly to bonds of similar duration regardless of issuer. Credit risk varies enormously across issuers and tends to rise sharply in recessions. When you read about "spread widening" in markets, that is the credit risk premium increasing for non-Treasury bonds relative to Treasuries.
Why bonds belong in portfolios
The historical case for bonds in a portfolio rests on two properties. First, they provide income — coupon payments arrive predictably and contribute to total return. Second, they have historically offered diversification against equities — when equities sell off in growth-driven recessions, Treasury bonds typically rally as the Fed cuts rates and capital flees to safety.
The 2022 experience was a stress test. Both equities and bonds fell simultaneously as inflation forced the Fed to hike aggressively. That is unusual but not unprecedented — it happens roughly once or twice per generation. For most other periods, the equity-bond correlation is negative, which is what makes the classic 60/40 portfolio robust.
Understanding bonds is the gateway to understanding macro markets generally. The yield curve, central bank policy, currency moves, and even equity valuation models all depend on the same arithmetic that governs a single bond. Master the basics, and the rest of the financial system suddenly makes sense.