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.[1]
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.
MECS has been influential in the theory of non-hierarchical markup systems, and is discussed in many articles by Michael Sperberg-McQueen and Claus Huitfeldt, among others.[2] [3]
Other systems for representing overlapping markup include the Layered Markup and Annotation Language, CLIX, JITTS, EARMARK and features of the Text Encoding Initiative and the Open Scripture Information Standard.