WILL Schuldenbremse enter the French or Italian languages the way “kindergarten” has become part of English? Perhaps. On December 5th the German chancellor, Angela Merkel, and the French president, Nicolas Sarkozy, agreed that the 17 euro-zone countries should insert a German-style “debt brake” into their constitutions. That, say Europe's first couple, will help prevent a recurrence of the crisis that threatens the euro's survival. The European Court of Justice should verify that all the national Schuldenbremsen pass muster.

Germany is not the first European country to have one. Switzerland introduced a debt brake in 2001. Poland caps its public debt at 60% of GDP, the limit it will have to observe if it joins the euro. But Germany's constitutional Schuldenbremse of 2009 is likely to be the model for the 15 euro-zone countries that do not yet have one. Under its provisions, the federal government must cut its structural deficit (ie, adjusted for the business cycle) to 0.35% of GDP by 2016; the 16 Länder (states) must eliminate theirs entirely by 2020. The idea is to slash public debt from a decadent 83% of GDP (close to France's rate) and never again to stray from the path of virtue. Spain adopted a slightly looser version in August; this week Austria passed a similar law (though not a constitutional amendment).

The German measure has broad popular support. But it also has its critics. Some economists object in principle, and warn of disaster if Europe follows suit. A bigger group is in favour but sees room for improvement. Peter Bofinger, one of five economic “wise men” who advise the German government, is in the first camp. The debt brake has two flaws, he argues. First, it breaks the “golden rule” that governments should be able to borrow to make investments that pay long-term dividends, such as in education. Second, it could choke off recoveries. The structural target allows deficits to rise when output falls below its potential. These should then be offset with surpluses during upswings. But this “assumes textbook-like economic cycles,” says Mr Bofinger. In real life, cycles are erratic. A recession followed by a weak recovery can shrink potential growth, which in turn could restrict the deficit spending needed to revive demand. Most economists think the brake needs tweaking, not ditching. Germany already had a constitutional golden rule, but that did not stop public debt from climbing (Mr Bofinger blames shocks like the financial crisis). The Schuldenbremse buttresses markets' confidence in Germany. The threat on December 5th by Standard & Poor's, a ratings agency, to downgrade Germany (along with other euro-zone countries) was triggered by the crisis, not by fiscal misbehaviour in Germany (see article). If natural or economic disaster strikes, deficit limits can be suspended. The extra debt can be paid off when conditions improve. But the Schuldenbremse is complicated and tricky to enforce, and will become no simpler if it spreads across Europe. Determining potential output is an occult discipline. The federal government thinks the output gap is negative this year (ie, the economy is below potential). The Bundesbank reckons it is positive. Spendthrift politicians could exploit such uncertainty.

Germany has yet to put its debt brake to the test. The federal government made things easier for itself by a generous calculation of last year's structural deficit, which is to be cut in equal annual steps to reach the 2016 target. Flush with cash, thanks to a strong economy, it has found room for giveaways to voters without falling foul of the brake. Civil servants will get a bigger Christmas bonus next year, for example. For the Länder, the 2020 deadline seems a long way off: 13 of them budgeted for increases in structural deficits this year, laments a study by RWI Essen, a research institute. A “stability council”, composed of federal and state ministers, has little power to sanction prodigals. Apparently, it is as toothless as the enforcers of European financial discipline.

Yet the debt brake is “clearly better” than the regime it replaced, says Heinz Gebhardt of RWI. Better still if governments beat the deficit targets, giving them scope for stimulus during slumps. Other countries may soon discover how German engineering works in their economies.