Suppose you have a 16:9 aspect ratio wide-format movie file and you want to make a version ideal for playing on a 4:3 device. Since you don't want to clip any of the image or distort the picture, so you want to make a letterbox version.
That is, you want a 4:3 movie with black bars at the top and bottom surrounding the moving image rectangle, which has true 16:9 dimensions.
Can you tell us how to do this in Windows or Linux, using free command-line or GUI tools.