When widescreen movies are shown on traditional 4:3 television sets, they are handled in two main ways. Most or all of the width of the film frame can be run across the width of the TV screen, which will result in a space above and below the film frame. These spaces are generally black, and give the impression of viewing a movie through a letterbox - and this technique is generally referred to as 'letterboxing'. You see the entire film frame, but it only occupies a centre strip of the screen - quite a thin strip if the original movie is a Cinemascope epic made in 2.35:1. The BBC's standard practice these days is to show movies in 16:9 for both analogue and digital viewers, which means that it is 'deep letterboxed' (quite wide black bars top and bottom) for viewers of analogue 4:3 sets. This is the case even if the film's original aspect ratio is wider than 16:9, in which case some cropping at the sides will occur.
The other approach is to run the movie at full television screen height, thus cutting off the parts of the image that lie outside the TV screen. This can be done dynamically, where the 'window' on the film frame represented by the TV set moves back and forth to cover the area of the frame that contains the bulk of the action - this is known as 'pan and scan' for that reason - or statically, simply showing the centre of the film frame that fits the TV screen - called 'centre cut-out'. In both of these cases, you lose a significant part of the film frame, and in the case of centre cut-out you may lose some of the action too.
It is commonly held by broadcasters that the image should fill the entire screen, because they believe - though there is little, if any, research to back this up - that viewers dislike the black bands that result from letterboxing. The fact is that on a 4:3 TV screen, you will see the full frame of a widescreen movie when it is letterboxed, albeit wide and thin top-to-bottom as it actually is; while if it is cropped to fill the screen, a significant part of the image will be lost. Thus it may be suggested that letterboxing is the 'best' way of presenting a widescreen movie for 4:3 viewers - at least you see what the director intended.
A Matter of Definition
Now, however, the situation is becoming much more complex, with the advent of digital 'standard definition' and 'high definition' television, and widescreen TV sets with an aspect ratio of 16:9. 'Standard Definition' or 'SD' means that the digital screen resolution is the same as it was for analogue transmission - that means 625 lines in the UK (PAL) and 525 lines in the US (NTSC).
We will assume the UK in this discussion unless otherwise indicated: the situation here is more complex, as there are both 4:3 and 16:9 broadcasts in SD (and, we shall see, 14:9!), while in the US, only 'High Definition' (HD) broadcasts are 16:9, which simplifies matters immensely.
The SD aspect ratio can be 4:3 as with analogue broadcasts, or 16:9. However, the 16:9 image as actually broadcast on the UK's digital television services (digital satellite - Sky; Digital Terrestrial Television - Freeview; or digital cable) contains the same amount of data as a standard 4:3 image: the 16:9 image is essentially 'squashed' into the 4:3 width and expanded again on reception. This is called '16:9 anamorphic' and is the same technique used in movie production to get widescreen images on to standard film, and for a similar purpose on DVD-Video discs. This means that the 16:9 image you see on an SD digital TV screen in the UK has rather less horizontal resolution than it does vertical. Luckily, you probably won't notice.
Three Aspect Ratios
There will always be a problem fitting 16:9 material to a 4:3 display, and in the UK, this has led to an additional complication. Analogue transmissions are 4:3 and will remain so until analogue television shuts down permanently during the next ten years. The vast majority of current programmes made or commissioned by the major networks are shot in 16:9 format, which is how you see them if you have a widescreen set and a digital television source such as Freeview, digital cable or digital satellite.
If you are watching analogue television on a 4:3 set, however, you will see a picture a little smaller than you are used to, with narrow black 'letterboxing' bars above and below the picture. This image has an aspect ratio of 14:9 and is a curiously British compromise between widescreen and 4:3. The edges of the widescreen picture are cropped, but the whole height of the original image is shown. Paradoxically, although the picture is not as tall as it was traditionally, you actually see more, but not the whole thing.
There are two other ways of presenting widescreen material on 4:3 sets, and the obvious one used for widescreen movies on television is, of course, showing most or all the width of the image on 4:3 sets, i.e. letterboxing. In British television production, the term 'letterboxing' is now generally used to describe 14:9 images on a 4:3 display, which has narrow black bars top and bottom and the left and right edges of the original picture missing, while the term 'deep letterboxing' is used to describe the display of the full widescreen image on a 4:3 display with resulting 'deep' black bars top and bottom. Allegedly, this is the least popular form of presentation on British television - and that is something to which we will return later.
The final method is centre cut-out, described earlier.
14:9 processing leads to a curious effect if you are watching a programme on a widescreen television that is receiving a 4:3 channel showing a re-run of a 16:9 show which has previously been processed for 4:3. The source material has been processed from 16:9 to 14:9, but it is being shown on a 4:3 channel, so the picture will 'float' with a black border all round.