DOLLARS and pesos cross the border between America and Mexico in greater numbers than ever. The $400 billion-worth of trade in 2010 made Mexico America's biggest trading partner after China and Canada. Greenbacks are so common south of the frontier that in some neighbourhoods peso coins are known as cuoras, a mispronunciation of “quarters”.

Lately the relationship between the currencies has been rocky. Between July and November the peso fell by 19% against the dollar, hitting its lowest level since the 2009 financial crisis. It has since bobbed back a little as prospects across the border have improved a tad. Nonetheless, its performance so far in the second half of this year has been the weakest of any Latin American currency.

Mexico is not the only emerging market with wobbly money. Nervousness about the global economy, thanks to the euro zone's debt crisis, has sent investors running from exotic currencies into safe, familiar ones. Big manufacturing countries everywhere, from South Korea to Poland, have suffered from worries about the impact of another recession on manufacturing, says Jose Wynne of Barclays Capital.

Three more forces have pulled the peso down. Mexico has stronger ties to America than any other big Latin American country: nearly 80% of its exports go there. (Brazil, by contrast, sends less than half its exports to rich countries.) Bad news from America hits the peso hard. Second, traders use the peso as a proxy hedge for the region, as it is the only Latin American currency to be traded around the clock. Some of the peso's biggest dips have happened overnight, during trading in Europe and Asia, notes Sergio Martin, chief economist at HSBC in Mexico.

Finally, Mexico's central bank is less willing than others to intervene. Whereas Brazil, Colombia and Peru have all sold dollars to support their currencies, the Banco de México has left the peso to the markets, even buying dollars now and then to build up its foreign reserves. That is changing: on November 29th Mexico's currency commission (made up of central bankers and finance-ministry officials) announced that the dollar-buying would end, and that up to $400m would be sold on days when the peso dipped by over 2%. The mechanism, last used in 2009, will temper volatility rather than defend the peso's value, the bank says. The peso rallied a little after the announcement.