Letterboxing is part of the process for melding the widescreen format of theatrical film and the boxed structure of television. The process is becoming dated, yet will still remain relevant for those using multiple formats in their projects.
Widescreen and Television
For most of the years of television there was a division between cinema and home viewing. The basic difference between the formats came from a shift that happened in the film industry in response to the release of television as a competitor. Previously, film had an aspect ratio that was much closer to television's 4:3, and when it was feared that consumers would just stay home and watch television for free they introduced the widescreen format to draw in audience members. This caused an issue when films were then shown on televisions, and this was especially a problem with the first release of the Betamax and the later release of VHS. This allowed newer films, which were almost all shown in this widescreen format, to be available relatively quickly from home. This created a huge market of consumers watching films that were prepared for a widescreen aspect ratio on a platform that was designed for 4:3 viewing.
This created a problem for those who were preparing the videos for later consumption. One of the solutions for this was called Pan and Scan. This process meant that the action of the screen was focused on and blown up so that the widescreen would be prepared for the 4:3 screen. The process cut out quite a bit of the frame, usually on the sides. That was a problem for many filmmakers who wanted to use the entire widescreen ratio and for viewers who wanted to see the film as it was originally presented. The solution to this began to be known as letterbox, and it also reflects the solution for conforming video prepared at 4:3 to the widescreen 16:9 ratio.
What letterboxing is refers to the presentation of a widescreen film or program in a form where the entire ratio can be seen on a 4:3 television. It does this by presenting the 16:9 aspect ratio through the middle of the screen with black boxes on the top and bottom. This does not utilize the entire television screen, but it does allow for the film to be seen in its original theatrical version. In the later days of the standard definition television format this became the standard as consumers decided that it was important to see films, especially classic and art films, in the form that they were intended to be seen. Most times when users are wondering what letterboxing is they are concerned about whether or not this presents an aspect ratio or not. Instead, it is just modifying one aspect ratio so it could have been shown on another.
Applying the Process
Many people who were actually producing video projects that were originally shot on the standard definition formats with the 4:3 aspect ratio would use letterboxing on their project. This was done for a number of reasons, and often since it was the preferred viewing method at that point. In this situation the answer to “what is letterboxing" was better answered by an elimination of some visual content, rather than the preservation of lost content from a widescreen transfer. To do this there is a cropping that is done on both the top and bottom of the screen until you match the 16:9 aspect ratio. Standard letterboxing is not an actual transfer of the video to a true widescreen format, but instead a modified standard definition where the black bars are present on the screen.
Now that widescreen HD televisions have become standard you will see less of a reason to have letterboxing at any level since the television mimics the aspect ratio of a film theater. Unfortunately, 4:3 footage from standard definition that is letterboxed is not always automatically viewed in widescreen on those televisions. Since that content is shown on the center area of the screen that can reflect the 4:3 aspect ratio it will include the letterboxing black bars, which were present on that video track. This actually makes things that were letterboxed for standard definition can cause problems, though you can set up DVD to focus in on the proper area. This means that it is entirely possible to prepare your originally letterboxed projects to be viewed properly on a widescreen television, but adjustments will have to be made. Today, most of the cameras that you will be filming on will be natively in 16:9 and you will already be dealing with this aspect ratio as the standard all through your production and post-production process. This is the default and you should not have to letterbox the content unless you are modifying older content that may have been created before the widescreen standardization for prosumer cameras. In this case you would have to present your project in your video editing program as a widescreen project and then bring in the 4:3 footage, conforming it to the correct format.
Letterboxing Image by Shane Burley.
Source: author's own experience.