The argument I will advance in this extended note on the United Kingdom Supreme Court’s decision in R (Miller) v Secretary of State for Exiting the European Union is that the decision and its aftermath can be usefully understood by reference to three fault lines: between form and substance (Part I); between the old constitution and the new constitution (Part II); and between political accountability and legal accountability (Part III). The decision and the academic debate the litigation provoked revealed that British lawyers are deeply divided about how to resolve important questions about the relationship between Parliament, the executive and the courts. And the legislative response to Miller reveals that the fault lines can operate differently in a political context than they do in a legal context (Part IV). Drawing the fault lines I have described and using them as analytical tools achieves three objectives. First, using the fault lines as analytical tools enhances the legal community’s understanding of Miller by placing the resolution of the issue relating to the triggering of Article 50 in a broader context. The litigation raised important questions about the way British lawyers understand their constitution, in particular, how to accommodate constitutional innovations such as EU membership, referendums and devolution. Second, illustrating that important tensions are to be found in the decision of the majority of the Court. For instance, on the juridical effect of triggering Article 50, substance trumped form, but when it came to the impact of triggering Article 50 on the devolution arrangements, form trumped substance; the ‘constitutional’ nature of the 1972 Act weighed heavily in the balance, but other constitutional innovations, such as referendums and devolution, exerted next to no weight at all; and although the executive’s political accountability as a matter of constitutional convention was deemed to be insufficient justification for avoiding parliamentary authorisation for the triggering of Article 50, the enforcement of constitutional conventions relating to devolution was left entirely to political actors. The fault lines thus expose important tensions in the reasoning of the Miller majority. Third, demonstrating that these fault lines are political as well as legal. The relationship between form and substance, old and new and political and legal shaped Parliament’s response to Miller just as much as it shaped the judgments of the Court and the first-instance courts. In the political arena, interestingly, form triumphed over substance, the referendum result carried decisive weight, and confidence about the effectiveness of conventional methods of parliamentary oversight of the executive outweighed concerns about the need for legal protection of the interests of individuals or Parliament. This does not represent an additional criticism of the judges. I mean only to highlight how the legislative response to Miller demonstrates that, under Britain’s constitutional arrangements, legal and political actors do not respond in the same way to the same stimuli. This phenomenon may not be unique to Britain, but if so, Miller is a striking example that ought to be of interest to constitutional lawyers in other jurisdictions.