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

XML

Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....

 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.

MECS has been influential in the theory of non-hierarchical markup systems, and is discussed in many articles by Michael Sperberg-McQueen

Michael Sperberg-McQueen

C. M. "Michael" Sperberg-McQueen is an American markup specialist. He was co-editor of the Extensible Markup Language 1.0 spec , and chair of the XML Schema working group....

 and Claus Huitfeldt, among others.

Other systems for representing overlapping markup include the Layered Markup and Annotation Language, CLIX

Clix

Clix can be:* CLIX , a version of UNIX developed by Intergraph* Clix , a Portuguese triple play brand* Axe Clix, a deodorant* iriver clix, rebrand of the iriver U10, a multimedia player...

, JITTS, and features of the Text Encoding Initiative

Text Encoding Initiative

The Text Encoding Initiative is a text-centric community of practice in the academic field of digital humanities. The community runs a mailing list, meetings and conference series, and maintains a technical standard, a wiki and a toolset....

 and the Open Scripture Information Standard

Open Scripture Information Standard

Open Scripture Information Standard is an XML application , that defines tags for marking up Bibles, theological commentaries, and other related literature....

.