At the heart of the +Italic Gauweiler -Italic case on the European Central Bank's (hereafter: the Bank) bond buying programme are two delicate and fundamental dimensions of the European constitutional order. +Superscript 1 -Superscript The first dimension concerns the autonomy of the European legal order, captured by the question of who ultimately decides on the validity and interpretation of EU law. The second constitutional dimension relates to the nature of the EU's economic and monetary union and concerns the question of what, if any, the role of the Bank is as a lender of last resort for sovereigns in the euro area