I don't know if this has happened to you, but there have been several times that I've uploaded a video to YouTube only to discover that it's stretched out of aspect ratio. That used to mean I'd end up having to upload the video again. It was the only thing that seemed to fix the problem. There are all sorts of creative ways to mess up a YouTube video, too. You could stretch it. You could squish it. You could upload a letterboxed 4:3 video into YouTube's 16:9 frame and make it look like there's a big black box around it.
As it turns out, you don't need to re-upload your video to fix any of those problems. You can take advantage of YouTube's hidden codes to force the video to display correctly. Note, this only works for videos you've uploaded into your own YouTube channel. You can't fix a video for someone else this way.
First, Some Definitions
4:3 - This is the aspect ratio of standard definition TVs in the US. The rectangle is four inches wide for every three inches tall. It's also the aspect ratio for old movies. If you've got home movies on VHS tape, this is probably the aspect ratio you'll find. But you'll also find this aspect ratio in a lot of computer monitors and even a few early HDTVs. Because it's a ratio, it's not a measurement of how many pixels or whether or not the video his high definition. It's just a measure of their proportion to each other in a rectangle.
16:9 - This is the aspect ratio of modern HDTVs. It's also commonly known as widescreen. For every sixteen inches wide a screen is, it's nine inches tall. In 2008, Google decided that this was the default resolution of all YouTube videos, so any video that doesn't fit into a 16:9 ratio must display either cropped or with bars. Again, this is not a measurement of pixels. Just the aspect ratio. There are a lot of standard definition video cameras that shoot in widescreen mode. Also note that lots of modern movie releases actually have an aspect ratio even wider than this. That's why they show up letterboxed on your screen.
Letterboxes and pillar-boxes. These are the black bars that show up on your TV or YouTube video to make room for a difference in aspect ratio. Letterboxes are horizontal stripes above and below a video and pillar-boxes are stripes on the side. If you upload a 4:3 video to YouTube, you'll see pillar-boxes on the screen.
Problems and How to Fix Them
All of these problems are going to be fixed by typing the secret code as a tag in the video. That's right. It's just a tag, and you can separate it with a comma and add other tags if you wish. When YouTube runs across one of these special tags, it knows the video needs to be displayed differently.
Video Stretched or Squished
If your video was 4:3 and is stretching to fill up the entire 16:9 video area, it's going to look off. Fix this problem by using the tag: yt:stretch=4:3
If your video has the opposite problem, and it's supposed to be a 16:9 video and is instead pillar-boxed and squished into a 4:3 space, you're going to use the opposite command: yt:stretch=16:9 Pretty easy, right?
Crop or Zoom
What happens if you've got a letterboxed 4:3 video you upload to YouTube? You end up with a video that has a giant black frame around all sides, that's what. You can fix this problem by cropping the video. You're zooming in, so if you do this to a normal video, you'll cut off some of the action, but if you do this with a framed in video, it will be perfect. The tag for this is: yt:crop=16:9