A variorum edition is gives not only the text the editor established, or the editor's own notes, but those of earlier critics as well. The Johns Hopkins Variorum Edition of Edmund Spenser, for instance, includes notes not only by that edition's editors, but by dozens of earlier critics, going back to the eighteenth century: for any passage in Spenser, the reader can see what earlier commentators have had to say about it.
Other variorum editions reproduce not critical commentary but textual variants and conjectural emendations proposed by many other editors. The Arden Shakespeare, for example, gives the readings in all the important early witnesses (the sixteenth- and seventeenth-century quartos and folios), and, when they're relevant, the readings proposed by editors from the eighteenth, nineteenth, and twentieth centuries.
Note: This guide is still in the early stages of development.
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