A continuous function from a circle to itself wraps the circle around itself a certain integer number of times. This is the degree of the map. A negative integer relates to the circle being wrapped in the opposite direction. This fact also shows the fundamental group of a circle is ℤ. Brouwer extended the notion of degree to continuous maps between compact oriented manifolds of equal dimensions, and again the number characterizes how many times the domain manifold wraps around the codomain manifold.