What are the dollar index, yields, Treasuries and FX?

Treasuries are US government bonds. Their yield is the return you earn holding them, and it is the number everything else is priced against. The dollar index, usually called DXY, measures the dollar against a basket of other major currencies. FX is the wider currency market where those exchange rates are set. They are not four independent markets. They are four views of the same thing, which is the price of holding dollars, so a move in one shows up in the others almost immediately.

Why does the rate differential matter more than the US yield alone?

Because an exchange rate is a comparison between two countries, so what matters is the gap between their yields rather than either level on its own. If US yields rise by 10 basis points and German yields rise by 20, the relative attraction of holding euros has improved even though US yields went up, and the euro can strengthen against the dollar on a day when US yields rose. This is why traders who watch only the US 10 year get confused by FX. Every currency pair has two sides and both are moving.

When does the higher yields stronger dollar rule break?

In risk off conditions, and this is the exception that matters most. When markets are frightened, investors buy US Treasuries for safety, which pushes yields down, and they also buy dollars for safety, which pushes the currency up. Yields fall and the dollar rises at the same time, which is the opposite of the usual relationship. The reason is that the dollar plays two roles at once. It is a carry currency, rewarded by higher rates, and it is the world's reserve and safe haven currency, bought in a crisis regardless of yield. Which role dominates depends on the regime.

How does all of this connect back to stocks?

Through the same discount rate that drives everything else. Rising yields compress equity valuations, and they usually lift the dollar, which separately hurts the reported earnings of large exporters because overseas revenue converts back into fewer dollars. So a sharp move higher in yields can hit equities twice. The useful habit is to read the combination rather than any single market. Yields up with the dollar up and stocks down is a coherent tightening story. Yields down with the dollar up and stocks down is a fear story, and those two call for very different responses.

How do I watch these correlations without a research desk?

Accept first that correlations are unstable. They shift with the regime and tend to break at exactly the moment you were relying on them, so treat any relationship as a current description rather than a rule. The practical approach is to check the relationship over more than one time window, since a link that holds over ninety days but not over thirty is telling you something changed recently. Also make sure you are comparing the right things: yields should be correlated on their daily change in basis points, not on percentage returns, because a yield is a level rather than a price.

How Do the Dollar, Yields and FX Correlate?

Q: Why do bond prices and yields move in opposite directions?

Because a bond pays a fixed amount and the price is what you pay to receive it. If you buy a bond paying 100 dollars a year for 2,000 dollars, your yield is 5%. If the price falls to 1,000 dollars, the same 100 dollars a year is now a 10% yield. Nothing about the bond changed, only what you paid. This is not a correlation or a tendency, it is arithmetic, and it means Treasuries rallying and yields falling are two descriptions of the same event rather than two separate things happening at once.

Q: How do yields and the dollar usually relate?

The usual relationship is positive: higher US yields tend to mean a stronger dollar. The logic is straightforward, because money moves toward better returns. If US government debt pays more, global investors need dollars to buy it, and that demand lifts the currency. The word usually is doing real work in that sentence though. It is a tendency rather than a law, it holds better over weeks than over minutes, and it breaks entirely in certain conditions covered further down.

Q: What is the dollar index actually made of?

Far less than people assume. The dollar index is dominated by the euro, which is roughly 58% of the basket, followed by the Japanese yen at around 14% and sterling at around 12%, with the Canadian dollar, Swedish krona and Swiss franc making up the remainder. That means DXY is largely a euro trade in reverse. It contains no Chinese yuan, no Mexican peso and no emerging market currencies at all, so it can be perfectly flat on a day when the dollar is moving sharply against the currencies that actually matter to a given trade.

Q: Which releases move all four at once?

The ones that change the expected path of interest rates. CPI is the biggest, because inflation feeds straight into rate expectations and therefore into yields, the dollar and every pair priced against it. Nonfarm payrolls is close behind. The Fed decision and press conference are their own category, and the meetings of other central banks matter just as much for the currency side, because they move the other half of the differential. On those days the four markets stop looking like separate screens and visibly move as one.

These look like four separate markets on four separate screens. They are closer to four views of one thing, which is the price of holding dollars. Once that clicks, most of the days that seemed contradictory stop being contradictory.

Ten short questions, answered one at a time.

1. What are these four things?

Treasuries. US government bonds.
Yields. The return you earn holding them, and the number everything else is priced against. Watch them on the rates page.
The dollar index, usually written DXY. The dollar measured against a basket of other major currencies.
FX. The wider currency market where those exchange rates get set.

The point of this post is that they are not four independent markets. A move in one shows up in the others almost immediately, because they are all describing the same underlying thing from different angles.

2. Why do bond prices and yields move opposite?

This trips up more people than anything else here, and it is not a correlation. It is arithmetic.

A bond pays a fixed amount. The price is what you pay to receive it. Buy a bond paying $100 a year for $2,000 and your yield is 5%. If the price falls to $1,000, that same $100 a year is now a 10% yield.

Nothing about the bond changed. Only what you paid for it. So "Treasuries rallied" and "yields fell" are two descriptions of one event, not two things that happened together. Once that is solid, the rest of this gets much easier.

3. How do yields and the dollar usually relate?

The usual relationship is positive: higher US yields tend to mean a stronger dollar.

The logic is simple enough. Money moves toward better returns. If US government debt pays more, global investors need dollars to buy it, and that demand lifts the currency.

The word usually is doing real work in that sentence. It is a tendency, not a law. It holds better over weeks than over minutes, and there is one condition where it inverts completely, which question seven covers.

4. Why does the differential matter more than the US yield?

Because an exchange rate is a comparison between two countries. What matters is the gap between their yields, not either level on its own.

Work through it. US yields rise 10 basis points. German yields rise 20. US yields went up, but the relative attraction of holding euros just improved, so the euro can strengthen against the dollar on a day when US yields rose.

That is why traders who watch only the US 10 year end up confused by FX. Every pair has two sides and both of them are moving. It also means other central banks matter as much as the Fed for currency, since they move the other half of the differential. Measured in basis points, the spread is the trade.

5. What is the dollar index actually made of?

Far less than most people assume, and this is worth knowing before you use it as a proxy for anything.

Euro: roughly 58% of the basket.
Japanese yen: around 14%.
Sterling: around 12%.
Canadian dollar, Swedish krona and Swiss franc: the remainder.

So DXY is largely a euro trade in reverse. It contains no Chinese yuan, no Mexican peso and no emerging market currencies whatsoever. It can sit perfectly flat on a day when the dollar is moving hard against the currency that actually matters to your position. Treat it as a broad gauge of the dollar against Europe and Japan, which is what it is, rather than as "the dollar".

6. Why does USDJPY track the US 10 year so closely?

Because it is the cleanest expression of a rate differential anywhere in the majors.

Japan has run far lower interest rates than the United States for a long time, so the gap is wide and persistent. That makes the yen the classic funding currency: borrow where money is cheap, invest where it pays more.

That trade depends directly on the spread between US and Japanese yields, so when the US 10 year moves, the incentive to hold the position changes immediately. USDJPY therefore reacts to US yields more mechanically than most pairs. It also unwinds fast when the gap narrows, which is why yen strength often arrives suddenly rather than gradually.

7. When does the rule break?

In risk off conditions, and this is the exception that matters most because it appears exactly when everything else is going wrong.

When markets are frightened, investors buy US Treasuries for safety, pushing yields down. They also buy dollars for safety, pushing the currency up. Yields fall and the dollar rises together, which is the opposite of the normal relationship.

The reason is that the dollar plays two roles at once:

A carry currency, rewarded when US rates are high.
The world's reserve and safe haven currency, bought in a crisis regardless of yield.

Which role dominates depends on the regime. That is not a flaw in the relationship, it is the relationship, and anyone treating "higher yields, stronger dollar" as a law will be badly wrong on precisely the days that count.

8. How does this connect back to stocks?

Through the same discount rate that drives everything else, plus one extra channel.

Rising yields compress equity valuations, which we covered in why the Treasury curve matters for stocks and futures. They also tend to lift the dollar, and a stronger dollar separately hurts the reported earnings of large exporters, because overseas revenue converts back into fewer dollars. A sharp move higher in yields can therefore hit equities twice.

The habit worth building is reading the combination rather than any single market:

Yields up, dollar up, stocks down. A coherent tightening story.
Yields down, dollar up, stocks down. A fear story.

Same equity move, completely different cause, and they call for different responses.

9. Which releases move all four at once?

The ones that change the expected path of interest rates.

CPI. The biggest, because inflation feeds straight into rate expectations and therefore into yields, the dollar and every pair priced against it.
Nonfarm payrolls. Close behind.
The Fed decision and press conference. Their own category.
Other central bank meetings. Just as important for the currency side, since they move the other half of the differential.

On those days the four markets stop looking like separate screens and visibly move as one. Whether a print was a genuine shock, rather than merely a headline, is a separate question answered by a surprise z-score and the calendar tells you when to expect them.

10. How do I watch these correlations?

Start by accepting that correlations are unstable. They shift with the regime and they tend to break at exactly the moment you were relying on them. Treat any relationship as a current description rather than a rule.

Two practical points follow.

Check more than one window

A link that holds over ninety days but not over thirty is telling you something changed recently. That difference is often more informative than either number by itself.

Compare the right things

Yields should be correlated on their daily change in basis points, not on percentage returns, because a yield is a level rather than a price. Running ratio maths on a near-zero-drift series distorts the result. Prices correlate on percentage return as normal. This is a common and quiet error in home built spreadsheets.

Where the terminal fits

Helious carries a cross-asset correlation matrix covering the UST 10 year and 5 year, the S&P 500, the dollar index, gold, crude, high yield credit and the VIX, over 30, 60, 90 and 120 day windows in one interactive card. It uses exactly the convention described above, correlating yields on their daily basis point change and prices on percentage return, so the numbers mean what you think they mean. Beside it sit the live curve, the calendar with alerts, every release scored the second it prints and a live news feed. It is $39.99 a month with a free tier, and the methodology page shows the workings.

Where to go next

The rates side: the rates page and the yield curve guide.
Why equities care: the Treasury curve for stocks and futures.
What moves the differential: CPI, the FOMC hub and the calendar.
The shape of the curve: steepening and flattening.
Another cross-asset read: VIX, VVIX and implied volatility.

Helious puts the curve, the dollar, the calendar and a cross-asset correlation matrix on one screen, so when four markets move together you can see whether the relationship still holds or has quietly changed. $39.99 a month with a free tier. Built by traders, for traders.

This post is general information and not financial advice. Index weightings and correlations change over time, so treat any figure here as approximate and check current values before relying on them. Trading involves substantial risk.

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Dollar Index, Yields, Treasuries and FX: How Do They All Correlate?