The research presented in this article has been supported by the European Research Council, through the Advanced grant n. 740611 , ‘Civil law, common law, customary law: consonance, divergence and transformation in Western Europe from the late eleventh to the thirteenth centuries’ (see http://clicme.wp.standrews.ac.uk).Throughout the Commentaries, Blackstone repeatedly availed himself of comparative legal history. Comparison allowed him to detect the invariable principles of legal experience and organise his systematic exposition of English law around them. This method proved crucial in Blackstone’s treatment of custom, as it allowed him to present the chief source of English common law by addressing the main questions concerning the nature...