Invisible ink letters inhabit a place in-between material support and the expectation of some kind of meaning. When critics talk about invisible ink, they tend to be concerned with similar things: how it was fabricated and when was it invented. This chapter briefly discusses the stereotypes that were put into play around secret writing in the early modern period, using this as a way to investigate women's participation in the Wars of the Three Kingdoms and to highlight the manner in which gendered narratives both constrained and facilitated this participation. It considers more broadly how invisible letters materialize and dematerialize the place of writing in early modern correspondence. In England, by 1677, chemically reactive inks had come to be called "sympathetical". The initial illegibility of letters written in invisible ink also brings to the fore the question of authorial authority. The most prevalent metaphor involving invisible ink in the early modern period concerned the evocation of sin.