Direct answer
EUR/USD (often written EUR USD or EURUSD) and USD/JPY matter in forex because they turn the same underlying currency exposure—US dollars—into two different exchange-rate relationships. A trader, analyst, or risk manager may care less about “a signal” and more about what changes when USD moves relative to the euro versus relative to the yen. In practice, this affects currency conversion, portfolio exposure, and how you estimate impacts when you can only observe quoted pair prices.
Mechanics and definition
A forex “currency pair” is the quoted exchange rate between two currencies. For a given quote:
- EUR/USD expresses how many US dollars you pay (or receive) for one euro.
- USD/JPY expresses how many Japanese yen you pay (or receive) for one US dollar.
Even without real-time data, you can see the shared structure: both rates include USD. That means USD’s movements influence both quotations, but the direction and magnitude of impact depend on which currency is the counter-currency and how you convert between them.
If you want to connect the two relationships in a consistent way, you must state assumptions. For example, assume you have some EUR exposure that you want to express in JPY terms. You can treat the two quoted rates as inputs to a conversion chain: EUR → USD using EUR/USD, then USD → JPY using USD/JPY. Any difference between “what you expected” and “what you get” can come from using different quote conventions, mismatched timestamps, or ignoring costs.
Evidence or example: what changes when the USD moves
Consider a simple scenario with made-up starting points (no claim about real markets):
- EUR/USD rises from 1.10 to 1.20 USD per EUR.
- USD/JPY rises from 150 to 160 JPY per USD.
If you combine these for a two-step conversion from EUR to JPY, you would multiply the “EUR to USD” rate by the “USD to JPY” rate. Under the assumption that the rates can be combined at the same moment, the implied EUR/JPY value would increase. However, in real use the two quotes may not update at exactly the same time, and your execution may occur with different spreads and slippage. So the conceptual link is stable, but the numeric result is not guaranteed.
A second comparison point is interpretability. If USD strengthens, EUR/USD typically moves one way (depending on the quote convention), while USD/JPY often moves another way because it measures USD against a different base currency. Both can matter because they describe distinct economic exposures: “euro vs USD” effects and “USD vs yen” effects.
Limitations and risks
- Relating pair moves to outcomes is assumption-heavy. To compute an impact in one currency from two quotes, you must assume you can combine the rates (same timestamp, compatible quote convention, consistent contract sizing). Without that, your conversion can be wrong.
- Costs and execution uncertainty can dominate. Bid/ask spreads, commission, and execution timing affect the effective price you actually trade, so small theoretical conversions can differ materially from realized results.
- Historical relationships do not establish future results. Even if EUR/USD and USD/JPY appear linked through USD, the way USD trades versus EUR and versus JPY can change with market conditions.
- Failure mode: hidden exposure direction. If you misunderstand which currency is base versus quote, you can interpret a rate move backwards (for example, confusing “USD per EUR” with “EUR per USD”). This can lead to consistent but wrong reasoning.
Verification and next question
To independently verify the practical relevance of EUR USD vs USD JPY, do a calculation with your own inputs:
- Pick a specific moment (or consistent time window) for EUR/USD and USD/JPY.
- Write down the quote format (what “per” means).
- Convert EUR exposure to JPY (or USD exposure to either currency) using explicit assumptions.
Then ask a second, testable question: How sensitive is your conversion to small changes in each input rate and to the timing difference between them? That sensitivity check often reveals whether the pair choice matters for your use case—or whether costs and timing matter more.