The Introduction includes an explanation of the use of the word “secret” in the title. It is taken from Walter Bagehot’s 1867 book The English Constitution, in which the author famously argued that “the efficient secret of the English Constitution may be described as the close union, the nearly complete fusion," the “singular approximation” of the executive and legislative powers. This is a suitable description of what happens in trilogues, as these indeed realize a singular approximation of the different European legislative institutions. It could therefore be claimed that trilogues are the “efficient secret” of the European constitution – that is to say, something not expressly regulated in the Treaties but crucial to the existence and functioning of the European political system.