Obsolescence can be defined in a number of ways, for example, going out of use, going out of production, becoming outmoded, or being replaced with something new.
Defining when a format goes out of use is difficult to do; people may continue to use it for many years after it goes out of production or is outmoded. Some formats may never truly go out of use and may continue to be used for niche applications.
Defining the date of when something goes out of production is usually easier. However, even then there are difficulties. Some computers for instance continued to be available as unsold stock for many years after they ceased production, or the manufacturing rights may have been sold on and another company made new examples under a different name for specific territories. In other cases, the devices are out of production, but new media are manufactured. Even now it is possible to buy new phonograph cylinders, and new games for the Atari 2600, and even as recently as 2015 it was possible to buy new Betamax tapes from Sony.
Replacing an older format with something new does not always mean the old one is obsolete. The older format may have a large established user base, or a large amount of content available. Digital Compact Cassette was supposed to replace the older analogue Compact Cassette but failed badly and was outlived by its predecessor. In other cases, the introduction of a new format caused rapid obsolescence of an old one; when DVD became available, people switched fairly quickly from VHS tape for video playback.
In terms of the dates for obsolescence given in the Museum, in most cases this is the date when production of either the devices to play the media, or the media itself, ceased (whichever came first). In some cases, where players or media are still available but the format is generally considered to have gone out of mainstream use, I’ve used an approximate date of when this happened.
Dates are given for the United Kingdom where known, which may differ from dates in other territories.