Representatives from Apple, Ireland and the EU appeared in front of the EU General Court in Luxembourg on Tuesday.

Apple is contesting a 2016 order by European Competition Commissioner Margrethe Vestager to pay back €13bn in taxes to Ireland, which was considered an illegal tax exemption. Ireland joined the appeal, siding with Apple.

The hearings will last through Wednesday although the ruling could take a couple of months. Even then, the ruling could be appealed further to the European Court of Justice.

The European Commission has argued that Apple’s effective corporate tax on European profits in 2016 was 0,005%. The Commission ordered Ireland to recover what it considers “illegal aid” in the form of unpaid taxes.

Apple’s lawyer, Daniel Beard, argued on Tuesday that the European Commission’s estimates essentially attribute all of the company’s income outside the US to Ireland; he also underscored that Apple is literally the biggest taxpayer in the world, paying an effective tax rate of 26%.

The European Commission’s legal representative, Richard Lyal, acknowledged the relative significance of Apple as a taxpayer, a fact that he called “… perfectly correct and perfectly irrelevant.”

Ireland considers the Commission’s ruling an infringement of national sovereignty.

Last year, the EU court denied US officials an opportunity to intervene in the tax challenge. Apple’s CEO, Tim Cook, has dismissed the case as “total political crap.”

Luxembourg is backing Ireland in the case, while Poland is supporting the commission.