Why the Dollar Rules Every Currency Trade

Episode summary: When trading minor currencies like the Thai Baht against the South African Rand, the dollar is always in the mix — even when it shouldn't be. This episode explores the mechanics of cross-pairs, why direct trades rarely set the price, and how the dollar's dominance shapes global currency markets. Learn why liquidity concentrates around the dollar, how arbitrage keeps exotic rates in check, and what it takes for a currency pair to break free from the dollar's gravitational pull.

Show Notes

### How the Dollar Shapes Every Currency Trade

Currency trading might seem like a straightforward exchange between two currencies, but in reality, the dollar is almost always involved — even when it's not part of the trade. This phenomenon is most evident in cross-pairs, where currencies like the Thai Baht (THB) and South African Rand (ZAR) are traded directly. Despite their bilateral economic ties, the exchange rate between these currencies is determined not by their direct trades but by their respective relationships with the dollar.

#### Cross-Pairs and the Dollar's Role A cross-pair is an exchange rate between two currencies that doesn't involve the dollar directly. However, the rate is calculated by dividing the dollar exchange rates of the two currencies. For example, to find the THB/ZAR rate, traders divide the USD/THB rate by the USD/ZAR rate. This method dominates even when direct trades between THB and ZAR occur.

The reason lies in liquidity. The dollar is involved in 88% of all global FX trades, creating deep, liquid markets for dollar pairs. When trading minor or exotic currencies, the dollar pairs (USD/THB and USD/ZAR) offer tighter spreads and better hedging options than direct trades. As a result, banks and market makers prefer to hedge their exposure through the dollar, reinforcing the dollar's central role.

#### Arbitrage and the Tether Effect Direct trades between minor currencies are possible, but they operate in the shadow of the cross-rate. Arbitrageurs ensure that any deviation between the direct rate and the calculated cross-rate is corrected instantly. High-frequency trading algorithms monitor these discrepancies and execute trades within milliseconds, pulling the direct rate back into alignment with the cross-rate. This arbitrage force acts as a tether, preventing direct trades from setting independent prices.

#### The Cost of Illiquidity Trading exotic cross-pairs comes with a premium. Spreads on pairs like THB/ZAR can be 10 to 20 times wider than those on dollar pairs, reflecting the cost of managing illiquid inventory. During periods of market stress, these spreads can widen dramatically, making direct trades even more expensive.

#### When Direct Markets Matter There are exceptions. Regional trade corridors with dense bilateral flows — like THB/MYR (Thai Baht/Malaysian Ringgit) — can develop independent liquidity. Factors like local banking infrastructure, regulatory frameworks, and central bank swap lines also play a role. However, even in these cases, the dollar's influence remains significant.

The dollar's dominance in FX markets is a product of network effects. Its liquidity attracts more participants, which in turn reinforces its liquidity. While alternatives like the euro have emerged, the dollar's role as the universal intermediary in cross-pair pricing remains unchallenged.

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