&fmt=18 is video with _480x360 pixels resolution (_4:3 ratio) recorded at _512 kbps

&fmt=22 is video with 1280×720 pixels resolution (16:9 ratio) recorded at 2000 kbps

&fmt=35 is video with _854x480 pixels resolution (16:9 ratio) recorded at _900 kbps

&fmt=18 is what’s considered high-quality

&fmt=22 is the standard for high-definition

&fmt=35 is the true version of high-quality.

via What are the differences between HQ18, HQ22, &, HQ35 standards? – Yahoo! Answers.

I’m using “DownloadHelper” to download videos from youtube, using the answers above.  It’s awesome to be able to download the 1080p video of a shuttle launch and watch it on my tv.

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