How does a stemma codicum show relationships among manuscripts?

In textual criticism, a stemma codicum is a family tree used to represent the genealogical relationships among surviving manuscripts of a literary work.12

The diagram functions by mapping how manuscripts descend from one another or from a common lost ancestor, known as the archetype. The construction of a stemma relies on the following principles:2

• Shared Variants: Critics identify relationships by analyzing shared variants, particularly errors and omissions, which suggest that certain manuscripts share a common source or lineage.2
• Vertical Transmission: The method assumes a "closed" tradition where each copyist followed a single model. By tracing these lines of descent, critics can discard manuscripts that merely repeat the testimony of others.2
• Reconstruction: By comparing the agreements of the remaining independent witnesses, the critic can infer the readings of lost intermediate manuscripts and, ultimately, the archetype.2

While theoretically powerful, the stemma is most effective in "closed" traditions. If a manuscript was copied from multiple sources—a process known as "horizontal" or "contaminated" transmission—the relationships become significantly more complex or impossible to map accurately.2