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This one is 16:9
And so is this one:
YouTube only tells us if the video is HD or not, so for now YouTube videos are presented as either 16:9 or 4:3.
This video is from Vimeo and has a super wide non-standard aspect ratio:
Vimeo supports pretty much any aspect ratio you give it. The Fluid Video Embeds plugin reads the dimensions from Vimeo’s API and then figures out the aspect ratio and preserves it.
This one is 2.39:1
More Anamorphic Videos