Calabi-Yau space is a type of mathematical space that enters into string theory, where the geometry of the Universe is held to consist of at least 10 dimensions – the four familiar dimensions of spacetime and six compact dimensions of Calabi-Yau space. These extra dimensions are so tightly curled up that they aren't noticed. Although the main application of Calabi-Yau spaces is in theoretical physics, they are also interesting from a purely mathematical standpoint.