BREXIT keeps on throwing up surprises. One came this week when the issue of a customs union with the European Union suddenly moved into the political spotlight. Another was the sight of business lobbies praising Jeremy Corbyn, Labour’s far-left leader, because he supported the idea of forming such a union, in hopes of uniting with rebel Tory MPs to defeat the government. Yet this technical-sounding matter was barely mentioned in the Brexit referendum—and few voters or even MPs really understand it.

The concept is old enough. The Zollverein, which abolished tariffs between German states in the 1830s, is a famous example. The EU’s customs union dates from its inception in 1957, though it was not completed until 1968. Like a free-trade area, a customs union scraps internal tariffs and quotas, but it adds a common external tariff. By definition, leaving the EU means leaving its customs union. But it does not preclude forming a new customs union with the EU, as Andorra, Monaco—and, on a bigger scale, Turkey—have done.

By getting rid of tariffs and quotas, a customs union facilitates trade. The EU’s customs union is essential to supply chains across Europe, with car parts and cabbages freely criss-crossing borders many times over. Unlike in a free-trade area, there are no costly rules-of-origin requirements to prove where products came from. And unlike membership of the EU’s single market, a customs union does not entail free movement of people, being directly subject to the European Court of Justice (ECJ) or paying into the EU budget.

Yet there are drawbacks to a customs union. As Turkey has found, being in one makes it impossible to strike free-trade deals in goods with other countries. Worse, it means that any such deals that the EU makes automatically open the Turkish market without giving Turkey reciprocal rights (cheap Mexican imports, for instance, can enter Turkey even though it has no free-trade deal with Mexico). Turkey’s customs union is only partial, excluding farm products, and it does not cover services. There are lengthy checks at the Turkish border with the EU.

Mr Corbyn tried to get around these problems by demanding not only a comprehensive customs union but also the right to do trade deals and to have a say in future EU free-trade negotiations. Brussels is unlikely to agree formally to this; indeed, the ECJ would refuse to allow it. But Britain, with greater clout than Turkey, might just be able to secure an informal right of consultation over EU trade deals.

The government still rules out any customs union at all. On February 27th Liam Fox, the international-trade secretary, labelled the notion a “sell-out” of British interests. Yet a customs union that covered only goods would allow free-trade deals in services, which Dr Fox energetically promotes. And the gains from possible future deals are smaller and further off than the immediate losses from leaving the EU and its free-trade deals with third countries.

This point was made by Dr Fox’s former permanent secretary, Sir Martin Donnelly, who likened his ex-boss’s position to rejecting a three-course meal now in favour of a packet of crisps later. The Treasury also frets that future gains are too remote to offset today’s losses. So does the Confederation of British Industry, which wants “a customs union for now”—that is, until future deals materialise. Another business lobby, the Institute of Directors, suggests a customs union that excludes most processed agricultural products and services, so these could be in free-trade deals.

The customs issue is bound up with another Brexit headache: averting a hard border with Ireland. The European Commission has released a draft legal text of the Article 50 withdrawal agreement, whose principles it agreed with Britain in December. The paper spells out the fallback option for avoiding a border, which is to keep Northern Ireland in a customs union and in full alignment with single-market rules. This implies that there might be a border in the Irish Sea, an idea rejected by the Democratic Unionists, who prop up Theresa May’s government. “No UK prime minister could ever agree to it,” she declared. If there can be no border on land or sea, a customs union could yet be forced on her.