What is vertical transmission in the copying of texts?

In textual criticism, vertical transmission refers to a process where a copyist relies on a single model or exemplar to produce a new copy of a text.1

In this model of transmission, the copyist is assumed to generate only variants or errors peculiar to their own work, rather than incorporating readings from multiple sources. Because the text descends in a straight line from one ancestor to the next, this type of tradition is described as "closed".1

This method is fundamental to the "genealogical" or "stemmatic" approach, which allows critics to construct a stemma codicum (a family tree of manuscripts) to reconstruct the earliest inferable state of a text, known as the archetype. This approach contrasts with "horizontal" transmission, also called "open" or "contaminated" transmission, where a copyist uses more than one exemplar or incorporates variants from multiple sources, making the reconstruction of the text's history significantly more complex.1