This isn’t simply because the FX market hates uncertainty. One reason is that the negotiations about the terms of Scotland’s departure from the UK might depress capital spending; most economists agree that uncertainty causes firms to delay irreversible investments such as location decisions. This - or the fear of it - would cause the Bank of England to postpone a rate rise and lower future interest rates naturally mean a weaker exchange rate.
Also, if Scotland uses sterling without the rest of the UK agreeing to a currency union, it would - as economists at the NIESR point out - need to run an external surplus. This would require a weaker pound in order to boost export revenues.
Thirdly, says Brian Hilliard at Societe Generale, a Scottish yes would increase the chances of the rest of the UK leaving the EU. This is because it would increase the chances of there being a Conservative government which has promised a referendum on exit, while removing the pro-EU Scots from the UK electorate.
Equally, though, says Trevor Greetham at Fidelity, a no vote on September 18 would cause a "strong bounce" in sterling by reducing these risks.
Such a bounce might be only temporary, however. Official figures this week showed that the UK has a big and growing trade deficit, which could eventually hit the pound. And the ECB’s decision to print money to buy asset-backed securities could weaken the euro. And sterling usually falls against the US dollar when the euro does so.
