The process of creating a full screen anamorphic image, also known as a "full height anamorphic" image, involves taking a 16x9, or 1.78:1 image and squeezing it horizontally to fit a 4x3, or 1.33:1 aspect ratio. The image is squeezed, on the X axis, by a factor of .8.
In the late 1980s, this process was done to accommodate analog, 16x9 televsion receivers which unsqueezed the image for presentation at the receiver. Today, this image is delivered, most commonly, on DVD's of theatrical motion pictures for viewing on 16x9 digital television receivers. Some conventional 4x3 analog televisions will "collapse" the 4x3 raster into a letterbox format to present the 16x9 image correctly.