9:16 is the ratio of a phone held upright, and every vertical video surface uses it: Reels, Stories, TikTok, YouTube Shorts, Snapchat, Spotlight. At a 1080 width it is 1920 tall; at 2160 wide it is 3840 tall. Those two numbers cover almost everything. YouTube's thumbnail guidance makes the second one explicit — 2160x3840 for Shorts, with a 640 px minimum height, and it warns that a vertical video carrying a 16:9 thumbnail will be replaced by an auto-generated 4:5 thumbnail on the home, explore and subscription feeds. That warning is the useful part: the platform will override a mismatched ratio, and when it does you lose control of the crop. The same logic applies sideways. Instagram keeps an organic photo at original resolution only between 1.91:1 and 3:4, and 9:16 is well outside that window, so a still image at 9:16 uploaded to a feed gets cropped — it belongs in a story or a Reel cover, not a feed post. Resize to 1080x1920 for the frame, 2160x3840 only for a thumbnail, and crop before you resize if the source is landscape.