Uploading Videos to YouTube without Letterboxes (Big Black Borders)

I was doing some occasional video editing, mostly using Adobe Premiere Elements.  Now and then, when I would upload a video to YouTube, it would have a letterbox.  That is, the video would be encased within a wide black rectangle.  This made the video smaller.  I didn't want it.  Getting rid of it was not easy.  This post presents a few notes in that direction.

One suggestion I encountered in a few different discussion threads was to add a certain tag to the Tags field available for each YouTube video.  The recommended tag was yt:crop=16:9.  I was supposed to type that into the Tags field, presumably separated from other tags by a comma, and that was supposed to expand the video.  I did find that this worked with one video, where I inserted that tag while the video was still uploading.  I found that it did not work with two other videos that were previoulsy uploaded, where I added the tag after the fact.  An alternative was to use yt:stretch=16:9.  That, too, failed with the previously uploaded videos.

I came across some Adobe suggestions on adjusting pixel and frame aspect ratios.  They didn't look terribly technical, and at some point I realized I would probably benefit from understanding them.

In the meantime, however, I found that I could sometimes resolve the issue by producing my video, in Premiere Elements, as an AVI, and then importing that AVI back into a new project.  Sometimes the re-importation would provoke Premiere Elements to ask if I wanted to correct my aspect ratio.  I didn't seem to lose any quality from this step, it didn't seem to require much additional time, and the MP4 that I would then output as my final project seemed to upload and display on YouTube without any letterbox problems.