9:20 aspect ratio
A 9:20 display shows 9 units of width for every 20 units of height — 0.45:1 in decimal form. This tall shape is the standard for phone screens held upright.
Phones held upright are the one place tall-and-narrow won. Stretching past the old 16:9 phone shape let manufacturers grow screen diagonals without making devices wider than a hand, so 20:9-class panels became the Android default. Video shot in landscape 16:9 occupies barely half of this shape — which is exactly why vertical video took over social platforms.