An autotransformer has a single continuous winding with sections acting as both primary and secondary, rather than two isolated windings. It provides voltage step-up or step-down with shared conductors, offering higher efficiency and lower material cost but reduced isolation. Autotransformers are used where galvanic isolation is not necessary, such as interconnecting systems with similar grounding or adjusting voltage levels in industrial and utility networks. Their design must carefully address fault levels and protection coordination.