4:3 aspect ratio
A 4:3 display shows 4 units of width for every 3 units of height — 1.33:1 in decimal form. Content mastered for 16:9 will show bars or crop on this shape.
The original screen shape: television, then every CRT monitor, then the first three iPad generations used 4:3. Video abandoned it for widescreen in the 2000s, but it never quite died — most budget projectors, plenty of industrial displays, and the iPad line (at nearly this shape) keep 4:3 content alive, which is why presentation software still offers 4:3 slide templates.