让我们从最基本的一点开始：离开欧盟意味着英国将退出关税同盟，这是欧盟28个成员国之间实施跨境自由贸易（并与外部其他国家实施共同对外关税​​）的基础。
Start with the basics. Leaving the EU means that the UK would exit the EU’s Customs Union, which is the basis for cross-border free trade among the EU’s 28 members (and establishes a common external tariff vis-à-vis third countries).