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A linear medium is any medium which is intended to be written to or accessed in a linear fashion, literally meaning in a line.
This means that the information is written to or read from the medium in a given order, so for example a book containing a novel is intended to be read from front to back, beginning to end, and is therefore a linear medium. It may be written in the same way, but would not necessarily need to be, to be considered a linear medium. A book containing an encyclopedia however is a non-linear medium, as it is not necessary for the articles to be accessed (or written) in any particular order. Even though both non-linear and linear media have perimeters to which they are restricted, linear media have a set path of how to get from point A to point B, whereas non-linear media do not.