Overlapping markup

In markup languages and the digital humanities, overlap occurs when a document has two or more structures that interact in a non-hierarchical manner.

A document with overlapping markup cannot be represented as a tree.

This is also known as concurrent markup.

Overlap happens, for instance, in poetry, where there may be a metrical structure of feet and lines; a linguistic structure of sentences and quotations; and a physical structure of volumes and pages and editorial annotations.

History

The problem of non-hierarchical structures in documents has been recognised since 1988; resolving it against the dominant paradigm of text as a single hierarchy (an ordered hierarchy of content objects or OHCO) was initially thought to be merely a technical issue, but has, in fact, proven much more difficult.

In 2008, Jeni Tennison identified markup overlap as "the main remaining problem area for markup technologists".

Markup overlap continues to be a primary issue in the digital study of theological texts in 2019, and is a major reason for the field retaining specialised markup formats—the Open Scripture Information Standard and the Theological Markup Language—rather than the inter-operable Text Encoding Initiative-based formats common to the rest of the digital humanities.

Properties and types

A distinction exists between schemes that allow non-contiguous overlap, and those that allow only contiguous overlap. Often, 'markup overlap' strictly means the latter.

Contiguous overlap can always be represented as a linear document with milestones (typically co-indexed start- and end-markers), without the need for fragmenting a (logical) component into multiple physical ones.