A ten-year government bond pays the same coupon next year as it did last year. Nothing in the contract changes when a central bank speaks, when an inflation print lands, or when a trading desk gets nervous. Yet the price of that bond is quoted differently every morning, and over a few months it can move more than the coupon pays in a year. The explanation is not mysterious, but it is mechanical, and getting comfortable with the mechanism is one of the most useful things a market watcher can do.

The seesaw, stated plainly

A bond is a loan with a fixed schedule. The issuer promises a set of cash payments: a coupon at intervals, then the face value back at maturity. Those payments are fixed in cash terms. What is not fixed is the price at which the bond changes hands before maturity — and the yield is simply the return you get if you buy at today's price and hold to maturity.

Because the cash flows don't move, the only way the yield can change is for the price to change, in the opposite direction. Pay less for the same stream of payments and your return rises. Pay more and your return falls. The market isn't setting a price and then calculating a yield out of curiosity; they are two views of one number.

A homely comparison helps. Imagine a contract that pays a fixed rent every year for a decade. If the going rate for such contracts rises, nobody will pay the old, higher price for yours — they will pay less, so that the fixed rent works out to the new going rate. The rent never changed. The price had to.

A worked example worth keeping in your head

Take a hypothetical ten-year bond with a 4% annual coupon and a face value of 100, trading at par. Suppose the yield the market demands for that issuer rises from 4% to 5%. The coupon is still 4 a year, but those payments now have to be discounted at 5%.

Discount the ten coupon payments at 5% and you get roughly 30.9. Discount the principal repayment the same way and you get about 61.4. The bond is worth about 92.3, not 100. A one-percentage-point rise in yield has cost roughly 7.7 points of price — more than a year and a half of coupon income.

Run it the other way. If the yield falls to 3%, the same bond is worth about 108.5. Notice the asymmetry: the gain from a one-point fall is slightly larger than the loss from a one-point rise. That is convexity, and it is why long-dated bonds reward the patient holder a little more than they punish the impatient one, all else equal.

Two things follow. First, the size of the price move depends on how far away the cash flows are: the same yield change hurts a thirty-year bond far more than a two-year note. Second, the loss is real to anyone marking the position to market, but it is not a default. The coupons still arrive on schedule.

Which yield are we actually talking about?

The word "yield" gets used loosely, and the looseness causes real confusion. Three versions turn up most often.

Coupon, current yield, yield to maturity

The coupon is the fixed rate on face value. It tells you what the bond paid when it was issued, which is history, not value. The current yield is the coupon divided by the price — quick to calculate, but it ignores both the capital gain or loss at maturity and the time value of money. The yield to maturity is the single discount rate that makes the present value of every remaining payment equal the price. It is the number worth quoting, because it is the only one that accounts for price, coupon and time remaining in one figure.

What a quoted yield quietly assumes

Yield to maturity assumes you hold to maturity and reinvest every coupon at that same yield. Neither is guaranteed. If rates fall, coupons get reinvested at lower rates; if rates rise, at higher ones. For callable bonds, ask for the yield to worst rather than the yield to maturity. For inflation-linked debt, distinguish the real yield from the nominal one. And for corporate bonds, separate the benchmark yield from the credit spread sitting on top of it — the two move for entirely different reasons.

What actually moves yields

Yields are a price for lending, so anything that shifts the balance of lenders and borrowers shifts them. The main forces:

• Policy rates and expectations. Central bank decisions hit the short end of the curve almost immediately. What matters more for longer maturities is the path the market expects from here.
• Inflation and inflation expectations. Investors demand compensation for expected inflation and for the risk that it surprises them. Rising inflation expectations tend to push nominal yields up, and the gap between nominal and inflation-linked yields is a live market measure of that expectation.
• Term premium and supply. Governments issue a lot of debt. When buyers need more encouragement to hold long-dated paper, they ask for a higher yield. Heavy issuance periods often coincide with steeper curves.
• Credit risk. For anything other than the safest sovereigns, the spread over the benchmark reflects the chance of not being paid back. Spreads widen in stress and compress in calm.
• Growth, risk appetite and flows. Weak data or a market scare sends money toward safety, which pushes yields down. Strong data does the reverse.
• Liquidity and positioning. Crowded trades unwind. A yield can gap a long way on flows before any fundamental number changes.

Duration: where portfolio risk actually lives

Duration is the bridge between a view on rates and the effect on a portfolio. In its simplest form it tells you roughly how much a bond's price moves for a one-percentage-point change in yield. A two-year bond at a 4% yield has a modified duration of about 1.9, so a one-point rise costs roughly 1.9% of price. A ten-year bond of the same coupon sits near 8. A thirty-year sits near 17. Same yield change, wildly different outcomes.

Duration is not the same as maturity. A low-coupon bond has more of its value arriving at the end, so it carries more duration than a high-coupon bond of the same maturity. A zero-coupon bond's duration equals its maturity, which is why it is the most rate-sensitive instrument in the toolbox.

For a portfolio, the practical consequences split two ways. If you already hold the bond and may need to sell before maturity, a rise in yields is a mark-to-market loss you have to live with. If you hold to maturity and the issuer pays, the loss is an opportunity cost — you are locked into an older, lower coupon while new buyers get better terms. Meanwhile, if your horizon is longer than your duration, higher yields are unambiguously helpful: your coupons and any maturing principal get reinvested at better rates. This is why a long-horizon saver and a trading desk can look at the same yield move and feel opposite things.

Bond funds deserve a note of their own. A fund's distribution yield rises when yields rise, because it is buying new paper at better rates. But its net asset value falls first. The income and the price do not move together, and judging a fund by its yield alone hides half the picture.

Read the curve, not just the point

A single yield describes one maturity. The curve describes the whole term structure, and its shape carries information that no single point can. Normally it slopes upward: lending for longer should pay more, to compensate for uncertainty. When short rates sit above long rates, the curve is inverted, which historically has often reflected expectations that policy will have to ease because growth is slowing.

Treat that as context, not a signal. The curve's shape blends rate expectations with the term premium, and the two can pull in opposite directions. An inversion tells you what the market is pricing, not what will happen. Watching how the curve changes — steepening, flattening, front-end versus long-end moves — is usually more informative than fixating on whether one spread has crossed zero.

A short checklist for market watchers

When a rates headline lands, run through these before forming a view:

1. Name the yield. Which issuer, which maturity, which measure? A move in two-year yields and a move in thirty-year yields are different stories.
2. Translate into duration terms. Multiply the yield change by the portfolio's duration to get a rough price impact. It is a coarse estimate, but it stops small-sounding moves from being dismissed.
3. Separate the level from the change. A 4.5% yield is neither high nor low without a reference point. Compare it with history and, more importantly, with inflation.
4. Look at real yields. Subtract expected inflation, or read the inflation-linked market directly. Nominal yields rising on higher inflation expectations is a very different situation from nominal yields rising on tighter policy.
5. Match horizon to duration. Money needed within a couple of years does not belong in long-duration paper, however attractive the yield looks.
6. Watch the whole curve. Note whether the move is at the front end, the long end, or both, and whether spreads are moving with it.

The relationship between price and yield is not a curiosity for exam questions. It is the transmission channel through which rate expectations reach every portfolio holding a fixed-income asset, and through which bond market moves spill into mortgage rates, corporate borrowing costs and equity valuations. Once the seesaw is intuitive, most of what passes for bond-market complexity becomes a matter of applying it carefully.

This is general information about how bond markets work, not investment advice. If a decision matters to your own finances, speak to a licensed adviser who can look at your full circumstances.

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