The ePUB file type is an open source format developed by the International Digital Publishing Forum (IDPF) . Based on XHTML, XML and CSS it works well with a number of software applications which has helped the popularity of the format grow. Despite this, there are a raft of rules that must be adhered to when creating an ePUB file which has attracted criticism of the format. The format allows DRM protection to be added to a file.
The ePUB format allows reflowable content. This means that the content fits the size and shape of the screen it is being displayed upon, in the same way that text on an HTML page changes depending on the size of the display. An ePUB file tends to consist of three elements: archive, metadata and content.
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The .tcr file type was developed by Psion for use with their Series 3 palmtop devices developed during the 1990s. The format was solely intended for use on the device, however there are certain eBook readers that will display .tcr files. This file format is however obsolete following the discontinuation of palmtop devices.