I still remember the first time I became aware of aspect ratio and the purpose of letterboxing. I was watching AMC (back when they actually played classic movies, you know), and they ran a little short piece between movies showing comparisons of The King and I, Lawrence of Arabia, and others in both pan and scan and letterboxed versions. I was floored by how much picture you lost when cutting the frame for pan and scan.
But that’s only the tip of the iceberg when it comes to aspect ratio. I knew there were a bunch of different widescreen formats in the 1950s and later, but didn’t know the technical specs or reasons for each one’s introduction, or even the way that 16×9 became the standard for HDTVs. Now I know all that and much more, thanks to this comprehensive and comprehensible video from Filmmaker IQ. It’s long, but it’s great stuff.