Chapter 1, ‘Unlocking Capacity’, contextualises and conceptualises the epistolarium in its thematic, stylistic, and comparative implications. The chapter begins by asking how we might engage with major collections of letters beyond traditional fact-mining. How might selected letters, by different hands, be explored as a site of consciousness and creativity? Appraising the leading editions of the letters of our five authors and artists, the chapter considers the traditional purposes of letter-editing. In pursuit of facticity, letter-editing traditionally subsidiarises the expressivity of everyday experience: Letterworlds tackles that critical blind spot. Arguing for an immersive reading of letters that can reveal a deeper human narrative, the chapter reviews pioneering approaches to the everyday across cultural sociology, epistolarity studies, and autobiography studies. The book’s major critical axes are introduced: sensory studies, theories of ‘negative community’, and everyday ethics. The material culture of letter-writing, from stationery supplies, via salutations and signatures, to postal networks, is explored in parallel with letterish creativity and modernist performativity.