Prepared by A-J | Author brand: 盡職生活｜阿J的普通人理財筆記
About the author/site · Vocus @WUCJ ↗ · Editorial policy
Page version: 2026-08-16 | Official or primary sources are preferred for material rules and figures. AI may assist structure, translation, and code, but is not used as the sole factual source.
About the author/site · Vocus @WUCJ ↗ · Editorial policy
Page version: 2026-08-16 | Official or primary sources are preferred for material rules and figures. AI may assist structure, translation, and code, but is not used as the sole factual source.
Key distinction: Market FX, a bank's posted rate, and your executable transaction rate can differ.
A cross rate uses a third currency
For example, USD/TWD and USD/JPY can be combined to derive a theoretical JPY/TWD relationship.
Keep quotation directions consistent
Inverting numerator and denominator produces a completely different result.
A theoretical cross rate is not necessarily executable
Banks apply buy/sell spreads, channels, and other costs, so real transactions use live bank quotes.
Use cross rates for understanding rather than live pricing
They are useful for checking currency relationships, not replacing executable FX quotes.
Primary / official sources
Posted rates change continuously. Actual transactions use the rate and fees shown by the executing bank, broker, or payment provider.