The deformation mechanisms in individual crystal structures have been extensively studied, such as the yielding phenomenon in face-centered-cubic (FCC) high-entropy alloys (HEAs) and the ductility in hexagonal close-packed (HCP) magnesium alloys. However, the physical connection of these mechanisms between the different crystal structures has rarely been investigated. For example, high-strength HEAs and ductile magnesium alloys both require low stacking fault energy (SFE), and the physical origin of this common feature is unclear. Here we provide a physical model for the SFE trend in FCC HEAs that connects the two phenomena. The physical model predicts the change of SFE based on elastic constants and geometrical parameters. This study paves the pathway to unifying the mechanics of close-packed materials.