The snake in the tunnel was a 1972 European exchange-rate arrangement. Learn what the snake and tunnel represented and why the system gave way to the EMS.
The snake in the tunnel was a European exchange-rate arrangement launched in April 1972 to keep participating currencies closer to one another while they also moved within wider limits against the US dollar. The snake represented the narrower spread among European currencies; the tunnel represented the wider dollar-based limits inherited from the Smithsonian exchange-rate framework.
The full “snake in the tunnel” phase was brief. When European countries suspended their margins against the dollar in March 1973, the tunnel disappeared, while a smaller group continued a jointly floating European currency snake until the European Monetary System began in 1979.
Under the Smithsonian framework, a currency could move up to 2.25% on either side of its dollar central rate. If one European currency was at the top of its dollar range while another was at the bottom, their potential spread could reach roughly 4.5%.
Participating European authorities agreed to keep the maximum spread among their currencies to 2.25%. On a chart, the group of European rates could move against the dollar inside the wider permitted corridor. The narrower moving group resembled a snake inside a tunnel.
flowchart TD
A["Smithsonian dollar limits<br/>create the wider tunnel"] --> B["European currencies agree<br/>to a narrower mutual spread"]
B --> C["Central banks intervene<br/>near the agreed limits"]
C --> D["Dollar margins end<br/>March 1973"]
D --> E["Currency snake continues<br/>with changing membership"]
E --> F["European Monetary System<br/>begins March 1979"]
The image is conceptual. The actual framework depended on bilateral rates, intervention arrangements, and central-bank agreements rather than a single tradeable “snake rate.”
Assume two currencies, A and B, have an agreed bilateral central rate of 10.0000 A per B. If a maximum 2.25% spread is represented symmetrically around that central rate for teaching purposes, each side is approximately 1.125%:
The simplified range is 9.8875 to 10.1125 A per B, a width of 0.2250, or 2.25% of the central rate.
This calculation explains the difference between a maximum bilateral spread of 2.25% and a plus/minus 2.25% margin, which would span approximately 4.5% of the central rate. Historical operating conventions should be taken from contemporary official records rather than reconstructed from this illustration.
| Component | Practical meaning |
|---|---|
| Bilateral limits | Participating currencies were kept within a narrower range against one another. |
| Dollar limits | Initially, each currency also remained within the wider Smithsonian margins against the dollar. |
| Intervention | Central banks bought or sold currencies as rates approached agreed boundaries. |
| Policy coordination | Interest rates, liquidity, reserves, and parity changes affected the ability to remain in the system. |
| Adjustable participation | Membership and central-rate relationships changed as economic and market pressure developed. |
The mechanism reduced day-to-day bilateral movement when it held, but it did not remove differences in inflation, productivity, fiscal policy, or external balances. Those differences could accumulate behind the exchange-rate limits and increase pressure for intervention or realignment.
| Date | Development | Analytical significance |
|---|---|---|
| December 1971 | Smithsonian Agreement established wider exchange-rate margins against the dollar | Created the dollar “tunnel” |
| 10 April 1972 | Participating central banks reached the Basel Agreement | Set the operating framework |
| 24 April 1972 | Intervention mechanism began | Start of the snake in the tunnel |
| March 1973 | Dollar margins were suspended | The tunnel ended; the European snake floated as a group |
| 1973-1978 | Membership changed and parities were adjusted | Demonstrated the strain created by divergent economies and shocks |
| 13 March 1979 | European Monetary System began operating | The EMS and ERM replaced the snake framework |
The history should not be reduced to a single “failure date.” The dollar-based tunnel ended in 1973, while the European currency snake continued in a narrower and changing form until the EMS began.
Countries experienced different inflation rates, growth conditions, and policy priorities. A fixed or narrow bilateral relationship becomes harder to defend when domestic price levels and interest-rate needs diverge.
Dollar instability and the 1970s oil shocks affected trade balances, inflation, and market confidence differently across participating economies.
Supporting a currency can require reserve sales, tighter liquidity, or higher interest rates. Those actions may conflict with domestic employment, credit, or financial-stability objectives.
Currencies entered, left, or changed relationships as pressure developed. A coordination framework is less credible when markets expect a participant to suspend intervention or adjust its parity.
Suppose a company in Country A agreed in 1972 to pay B 5 million to a supplier in Country B after 90 days. At a central rate of 10.0000 A per B, the expected cost was:
B 5,000,000 x 10.0000 = A 50,000,000.
If the bilateral rate reached the simplified upper boundary of 10.1125, the invoice would cost:
B 5,000,000 x 10.1125 = A 50,562,500.
The difference is A 562,500, even though the exchange rate remains within the illustrative narrow range. The company also faces a separate scenario in which a parity is changed or participation is suspended. That event could produce a move beyond the prior band and should not be treated as impossible because the system had intervention rules.
This hypothetical example excludes spreads, forward points, fees, controls, settlement constraints, and accounting treatment.
The snake showed both the attraction and difficulty of European exchange-rate cooperation. Reduced bilateral volatility could support trade and integration, but narrow bands required compatible policies and credible intervention.
The European Monetary System began in 1979 with a more formal structure. Its core Exchange Rate Mechanism used central rates connected to the European Currency Unit and a bilateral parity grid.
The institutional lineage is important, but the systems were not identical. Analysts should not use snake limits, ERM I bands, or modern ERM II rules interchangeably.
This article is for financial education only. Historical exchange-rate rules should not be used as current trading, valuation, legal, or investment guidance.