OS X El Capitan (version 10.11) is the twelfth release of macOS, shipped on September 30, 2015 — a refinement year named, pointedly, for a rock inside Yosemite. Its additions were quality-of-life (Split View full-screen pairing, a shake-to-find cursor, pinned Safari tabs, natural-language Spotlight) and its foundations consequential: Metal, Apple's low-overhead graphics API, replaced OpenGL at the system's heart, and System Integrity Protection ("rootless") walled system files off even from administrators.

Split View; Mission Control refinements; Metal on the Mac; System Integrity Protection; Spotlight natural language and live content; Safari pinned tabs and tab audio muting; Notes overhaul; Photos editing extensions; and a systemwide San Francisco font.

Metal became the graphics substrate of every Apple platform (and the lever that later eased the Apple-silicon transition), while SIP set the security template — protect the system from everything, including its owner, with an escape hatch — that hardened progressively into today's sealed system volume.