TEI is an XML-based markup language that enable scholars to store, analyze, and share humanities textual information. The Text Encoding Initiative (TEI) Consortium is a organization that develops the standards for text encoding in the TEI encoding language. The TEI Consortium developed and continually revises The Guidelines to Text Encoding and Interchange, the guidelines for text encoding in TEI.

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The diagram below, borrowed from the Women Writers Project, shows how TEI can be used in a number of different scholarly applications. While all these different uses share a common core of certain elements of TEI, the TEI is also flexible enough that is can cater to each unique type of project or document.

Image: https://wwp.northeastern.edu/outreach/seminars/_current/presentations/tei_intro/TEI_introduction_05.xhtml​ 

The process of altering the TEI Schema to meet your needs is called customization. This process can include:

• Adding or omitting elements
• Changing names of elements or attributes
• Restricting the values of attributes (possibly by creating a controlled vocabulary)

Customized Schemas are what you use to validate your TEI document. Using most editors, like oXygen, if you specify that you are working you with a specific Schema, it will indicate if you are violating the Schema, and even restrict the possible elements and attributes that are possible in a given situation (see the previous section on well-formedness and validity for more information).

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