21:9 aspect ratio
A 21:9 display shows 21 units of width for every 9 units of height — 2.33:1 in decimal form. Content mastered for 16:9 will show bars or crop on this shape.
Marketing shorthand rather than exact math: so-called 21:9 panels actually range from 2.33:1 to 2.39:1, echoing cinema's widescreen formats. In practice ultrawides replace a dual-monitor setup with one seamless canvas — two documents side by side, or a wraparound view in games that support it. Films shot in cinematic widescreen fill the panel gloriously; regular 16:9 video shows pillarbox bars.