In any event, without further ado, let's allow the real expert to explain things:
What constitutes a cento is probably most familiar in the shape of a piece of poetry consisting entirely, or almost entirely, of lines or parts of lines taken from famous poems (the Iliad and Odyssey, or the Aeneid) which are strung together in such as to form a new poetic composition.1 From the Hellenistic period to the eighteenth and even the nineteenth century CE this was a highly appreciated genre. To be successful, such a composition not only presupposes a 'poet' who is entirely familiar with his 'source' but also a public which knows its Homer, Virgil or Petrarch by heart or at least very well. Those who do not have the necessary literary sophistication are fooled and may believe the new piece really is by Virgil or by Homer, but this is more their own fault than that of the author of the cento. For the connoisseur it is fun to read Ausonius' Cento nuptialis, especially the pornographic section XVII viii, 216-7, or the Medea extant in two manuscripts which may be the one mentioned by Tert. De praescr. haer. 39.3 -4 both of which consist of quotations from Virgil serving a totally different purpose, fun precisely because the 'old' lines or half-lines in their new order and setting are provided with other and recherche meanings. The tragedy Christus patiens that goes under the name of Gregory Nazianzen has been fabricated from bits and pieces for the most part taken from Euripides, with additions from Sophocles and Lycophron and even one line from Homer; here the pagan material is made to serve a holy purpose. . Many more examples could be cited, such as the Homerocentones of the empress Eudocia Augusta (fifth cent. CE) and others edited by Ludwich the majority of those that are extant being by Christian authors both Latin and Greek.
A person composing a cento must have an 'argument' or 'plot' (ὑπόθεσις), around which to weave the materials used by him. In fact, this argument or plot and the actual choices that are made from the available material are his only personal contribution; the topic chosen may be an already existing one, as in the case of eg the Christus pattens or of the Medea, but need not be so.
One may also compose a cento consisting of non-poetic material (I note in passing that the prose cento, although explicitly mentioned by Eustathius, has been little studied). Such a patchwork may serve a plurality of purposes and may differ from the poetic cento in that it may explicitly mention the source, or sources, used, or at least some of them. Indeed, it is not always easy to distinguish between a cento in the proper sense of the word and a string of laudationes that have been collected to serve a particular purpose. One may for instance select and arrange quotations from the works of Zeno of Citium, or of Epicurus, in a tendentious way in order to prove these philosophers immoral.4 What one finds in such cases is not a cento in the sense of the recycling of existing materials for a new and different purpose, as it would be if one eg were to compose a treatise on gardening by stringing together in a sophisticated way quotations from Epicurus's works. But it is a patchwork in the sense that the medley of quotations both is, and is not, Epicurean, or Zenonean; by using existing Epicurean, or Zenonean, materials and by conveying the image that suits his polemical purpose the opponent of Zeno, or Epicurus, produces a pseudo-Zeno, or a pseudo-Epicurus, or at most a mezzo-Zeno or mezzo-Epicurus.
A further variety is a string of quotations collected not from one writer or one work but from a plurality of authors, in order to illustrate, or support, a point one wishes to make. In such cases the passages (in both verse and prose) that are quoted may be entrusted with new meanings, or at least acquire meanings not intended by their original authors, and as a rule the mosaic is glued together with interpretive comments which support and expound the view of the person who has made, or adapted, the collection of quotations. A quite unsophisticated example of such a cento is ps.Justin, De monarchia, consisting of a string of thirty quotations from the poets and one from Plato, with brief foreword and afterword and a few interspersed comments. Another variety, again, is the string of biblical quotations, put together in order to underpin a theological point or settle an exegetical question. In the works of Philo of Alexandria, for instance, examples of such chains abound; what is more, in the running commentaries woven around these biblical pericopes or formulas Philo often enough subjoins quotations, or reminiscences, of sayings and ideas of Greek philosophers, most of the time without giving their names. To divulge a name is to appeal to an authority for a definite purpose, for instance to prove that it was Heraclitus who took over an idea from Moses. The educated section of Philo's public may have known what and whom he was using or referring to, when he went about his business without citing chapter and verse, but others may not. [Jaap Mansfeld, Heresiography in context: Hippolytus' Elenchos as a source for Greek philosophy p. 160 - 161]
