16:9 aspect ratio
A 16:9 display shows 16 units of width for every 9 units of height — 1.78:1 in decimal form. It's the standard shape for monitors, TVs, and video.
The default shape of the modern world: HDTV settled on 16:9 in the 1990s as a compromise between old TV and cinema formats, and monitors, laptops, phones (held sideways), and video platforms followed. Its dominance is self-reinforcing — content is mastered for it, so displays adopt it, so more content is mastered for it. Every other ratio on this site is defined by how it differs from 16:9.