In the joined appeals of Cavendish Square Holding BV v Talal El Makdessi, ParkingEye Limited v Beavis [2015] UKSC 67; [2016] AC 1172 (Makdessi), the Supreme Court reformed the modern penalty clause doctrine, including by emphasising that it is available only in relation to secondary obligations, which must be classified as a matter of substance not form. This article considers this threshold test and the issue of so-called “disguised penalties” in English law following that decision.