The threshold concepts approach to student learning and curriculum design now informs an empirical research base comprising over 170 disciplinary and professional contexts. It draws extensively on the notion of troublesomeness in a "liminal" space of learning. The latter is a transformative state in the process of learning in which there is a reformulation of the learner's meaning frame and an accompanying shift in the learner's ontology or subjectivity. Within the extensive literature on threshold concepts, however, the notion of liminal space has remained relatively ill-defined. This paper explores this spatial metaphor to help clarify the difficulties that some teachers observe in the classroom in regard to their students' understanding. It employs a novel and distinctive approach drawn from semiotic theory to provide some explanatory insight into learning within the liminal space and render it more open to analysis. The paper develops its argument through four distinct phases. Firstly it explores the spatial metaphor of liminality to gain further purchase on the nature of this transformative space. The second section introduces semiotic theory and indicates how this will be used through a series of graphical and visual devices to render the liminal space more open to analysis. The third section then employs semiotic analysis to nine dimensions of pedagogical content knowledge to gain further insight into what may characterise student conceptual difficulty within the liminal state. The fourth and concluding section emphasises the role of context in conceptual discrimination before advocating a transactional curriculum inquiry approach to future research in this field.