-aspect, -monitoraspect

What are the differences & uses of the -aspect and -monitoraspect arguments for mplayer? I’m just trying to figure out how to not have my movies stretched on my 16:9 HDTV. Some simple documentation would be a nice addition.

I’m not an expert but I believe -aspect defines the aspect ratio of the actual content (e.g. the movie or TV show) whereas -monitoraspect defines the aspect ratio of the monitor on which that content will be displayed.

I used to use -aspect with something like 19:17 but now I used -monitoraspect and 16:9