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MECS is the Multi-Element Code System, a markup system developed by the Wittgenstein Archives at the University of Bergen. It is very similar to SGML and XML except that it allows elements to overlap.
MECS supports limited overlap, in that a given type of element (for example a quotation), may overlap other types, but no element may overlap another of the same type.
Other systems for representing overlapping markup include the Layered Markup and Annotation Language, CLIX, JITTS, and features of the Text Encoding Initiative and the Open Scripture Information Standard.