Stick–slipStick-slip instability instabilityDynamic stiction is widely known as a friction-induced vibration across scales caused by the repetition of static and kinetic friction. Nowadays, its elementary process, the stick-to-slip transition, is taught to high school students as a fundamental physical phenomenon in the early stage of classical mechanics. However, based on a purely mechanical sliding model that considered an in-plane angular misalignment, Nakano and Popov recently proposed a different interpretation of stick–slip instability, in which the stick state is made (not by magnitude change in static friction but) by direction change in kinetic friction [Nakano and Popov, Phys. Rev. E, 102, 063,001 (2020)]. This unique quasi-stick state, termed dynamic stiction, surprisingly explains slow-slip events that many researchers previously observed in their precise experiments. With numerical simulations, we here demonstrate the nature of stick–slip instability in a single-degree-of-freedom sliding system with an in-plane angular misalignment. We compare stick–slip instability with dynamic stiction to stick–slip instability with conventional stiction, discussing similarities and differences between the two interpretations. Their contrast raises an essential question in tribology: What is static friction?