In the multi-layered system of international human rights law, the European Court of Human Rights (ECtHR) uses not only its own case-law but also external instruments (EI s) when interpreting the European Convention on Human Rights (ECHR). This article explores the horizontal judicial borrowing practice of the Court until 2020. The landscape includes a wide range of different sources, but with a clear preference for Council of Europe instruments and secondly United Nations sources. Bridging what EIs are cited with how they are used, a case-study is performed investigating the functions that the EIs serve under Article 3 ECHR. Although it is uncommon, explicitly relying on EI s suggests that the Court views it to be legitimate. Yet, it is not uncontroversial. The Court’s motivations in citing EI s can be understood as justified from a universal human rights perspective, and susceptible to criticism from the perspective that it could undermine State sovereignty.