The guidelines of the Text Encoding Initiative make recommendations about suitable ways of representing those features of textual resources which need to be identified explicitly in order to facilitate processing by computer programs. They specify a set of XML tags in order to mark the textual metadata, text structure, relationship between images and transcriptions and other features of interest. The guidelines both describe the formal TEI encoding scheme, and document the TEI markup vocabulary. In their 30 years community driven development they have developed to a de-facto standard in the production of textual data in the humanities.