GIVEN how few voters enjoy paying them, politicians rarely trumpet the advent of new taxes. But the passage of a new goods-and-services tax (GST) in India’s upper house on August 3rd is a deserved exception. Well over a decade in the making, the new value-added tax promises to subsume India’s miasma of local and national levies into a single payment, thus unifying the country’s 29 states and 1.3 billion people into a common market for the first time. The government of Narendra Modi, never averse to over-hyping what turn out to be modest policy tweaks, has enacted its most important reform to date.

Few countries are fiddlier than India when it comes to paying taxes; the World Bank ranks it 157th out of 189 for simplicity. Both the central government and powerful state legislatures impose a dizzying array of charges. Because the rates differ between states, making stuff in one and selling it in another is often harder within India than it is in trade blocs such as NAFTA or the European Union. Queues of lorries idle at India’s state boundaries much in the same way they do at international borders.

That should change with the GST, essentially an agreement among all states to charge the same (still to be decided) indirect tax rates. Businesses are thrilled at the idea of being able to distribute their products from a single warehouse, say, rather than replicating supply chains in each state. Thick, exception-riddled tax codes—car sales are liable to six different levies at various rates, depending on the length of the vehicle, engine size and ground clearance, for example—are to be replaced with a single GST rate to be applied to all goods and services.

Better yet, the GST will be due on the basis of value added. That avoids businesses being thwacked by taxes on the entire value of the products they buy and sell rather than just the value they create—a situation that often made it cheaper to import stuff rather than make it locally. Just as importantly, by requiring businesses to document the prices at which they buy inputs and sell products (unless they wish to pay higher taxes), it will force vast swathes of the economy into the reach of the taxman.

Economists and technocrats have long backed the GST, which they think could boost economic output by 1-2 percentage points a year. Their calls were insufficient to overcome India’s petty politics: GST proposals stalled under governments of left and right since it was first mooted in 2000. Mr Modi, as chief minister of Gujarat state until 2014, helped thwart the previous government’s GST plans and has faced retaliatory obstructionism since. A committee of various states’ finance ministers helped convince regional parties in the upper house, which Mr Modi’s government does not control, to clear the blockage.

Because the tax overhaul requires a new amendment to the constitution, and therefore the backing of at least 15 state legislatures, it will take several months to enact. Few expect it to be derailed, but a deadline of April 2017 seems unlikely to be met. Though efforts to water down the bill (for example by exempting petrol) appear to have been overcome, its precise workings remain undecided. Even the GST rate is unknown; a government study mooted 17-18% but some states (which will receive the cash raised) would like it to be higher.

Such nitty-gritty will be fought over in the “GST council”, a novel body which will represent both state and federal executive branches but looks likely to be dominated by ministers sitting in New Delhi. Arvind Subramanian, the government’s chief economic adviser, calls the whole construct “a voluntary pooling of sovereignty in the name of co-operative federalism”, borrowing freely from the lexicon once used by the builders of the EU’s common market a generation ago. Such projects do occasionally run into bouts of difficulty.

Indeed, the new council and the tax it will administer go against a recent trend for decentralising power from New Delhi to the various state capitals. Powerful chief ministers sitting in the provinces will be more dependent on revenue collected federally and less on purse-strings they control themselves. Money will shift from (richer) states that make things towards (poorer) ones that consume them, too. The advent of a single tax to rule them all may come to shape Indian politics as much as it does the economy.