1:1Square
4:3Classic TV, iPad, older monitors
3:235mm photo, many DSLRs
16:10MacBook & many laptop displays
16:9HD video, YouTube, most monitors
21:9Ultrawide monitors, cinematic
9:16Vertical video
3:4Portrait photo
2:3Portrait 35mm, posters
5:4Large-format photo, 1280×1024
1.91:1Open Graph / social link previews
2.39:1Anamorphic widescreen film