Supplementary material is a broad category generally characterised as material added to the end of a text that was distinct from the main body but intended to reinforce the central argument. This material could include appendices, epilogues, afterwards, or coda. Supplementary material could feasibly stand on its own and was more substantive than other forms of backmatter. A useful contrast would be the index, which allowed a reader to engage with the central argument by finding key terms, but only functioned in relation to the specific text to which it referred. Meanwhile, the substantive nature of supplementary material generally required that its subordination to the main body was denoted either explicitly through headings or implied by its position in the text itself.
A useful example is the compendium edition of A Looking-Glass for Presbyterians. The edition brought together a number of different titles, some of which had appeared independently, into one continuously numbered text. Looking-Glass for Presbyterians refuted the Quaker Unmask'd conspiracy and aimed to demonstrate that the Paxton Boys were part of a long-running Presbyterian conspiracy against monarchical government. There were two titles, "Looking-Glass Numb I" and "Looking-Glass Numb II," that constituted the main body of the text, which argued that from Charles I to George III the Presbyterians had worked to undermine the British crown.
These two titles were then followed by three discrete titles under a larger "APPENDIX" heading. The three further titles each supported the idea there was a Presbyterian conspiracy, but did so using satirical and fictional forms. Two of these titles, "Substance of a Council" and "A Letter from a Gentleman in Transilvania," had appeared as separate texts. By subordinating the satirical works to the main body, the text effectively asked readers to consider the serious argument of Looking Glass in light of fictional evidence and analysis.
Another example of supplementary material is the coda to A Letter to Batista Angeloni. Letter was a copy of John Shebbeare's 1756 satire of English mores. The fictional letter dismissed positive reports about the Quakers from real authors such as Voltaire. Instead, the persona of Angeloni, an imagined Jesuit travelling in England, claimed that Quaker religious beliefs were a sham. He reported that Quakers acted in self-interest, refusing to help the nation or community at large. The piece concluded by pointing to the impracticality of Quaker government in Pennsylvania. The piece was written at the beginning of the Seven Years' War, which served as the locus of many of the issues in the Paxton Boys debate. Shebbeare predicted that Pennsylvania would fail to confront the French because Quakers could not oversee a war, and in that respect, he was prescient. However, Shebbeare also said that the Quakers had managed to avoid war with the Native Americans because Indians could be bought off with small gifts, which Pemberton's Friendly Association had demonstrated was not true. After Shebbeare's closing remarks, the 1764 edition inserted another poem as a coda.
A coda is a concluding statement that helps readers make sense of what happened in the main body of the text. The title page made clear that it reproduced Shebbeare's Letter "to which is added The Cloven-Foot discovered," thereby making the Cloven-Foot poem supplementary material. This also highlights another advantage in to reproducing the whole printed text: title pages help researchers parse the subordination of material within a text. The coda to Letter picked up on Shebbeare's prediction about Pennsylvanian government to show the violent implications of giving the Indians small gifts. In effect, the coda brought Shebbeare's satire up to date in 1764.
Supplementary material encouraged readers to think about the hierarchy of material within a given text, which reframed the main body. Appendices and coda prompted readers to engage with the main body of the text twice, once as part of the core reading experience and a second time to consider how supplementary material responded to the central argument.