This book presents four case studies that interrogate how German fifteenth-century painted triptychs engage with, and ultimately blur, various boundaries. Some of the boundaries are internal to the triptych format, for example, transgressed frames between narrative scenes on triptych interiors, or interconnections between imagery on triptych interiors and exteriors. Other blurred boundaries are regional ones between the Netherlands and Cologne; metaphysical ones between heaven and earth; and artistic distinctions between the media of painting and sculpture. The book’s case studies—which shed new light on Conrad von Soest, Stefan Lochner, and the Master of the St. Bartholomew Altarpiece—illuminate the importance of German fifteenth-century painting, while providing a fresh assessment of relations between German triptychs and their more famous Netherlandish counterparts. The case studies also demonstrate the value of probing Medialität, that is, the implications of format and medium for generating meaning. A coda assesses the triptych in the age of Dürer.