The TEI encoding scheme consists of number of modules ('tagsets') or DTD fragments. The DTD fragments from which the main TEI DTD is constructed are classified as follows:
standard components of the TEI main DTD in all its forms; these are always included without any special action by the encoder;
basic building blocks for specific text types; exactly one base must be selected by the encoder (unless one of the combined bases is used);
extra tags useful for particular purposes. All additional tag sets are compatible with all bases and with each other; an encoder may therefore add them to the selected base in any combination desired.