In this lecture we put all the pieces together and prove the Duality Theorem that relates the Čech cohomology of a nested pair of compact subspaces of an $n$-dimensional oriented topological manifold and the relative singular homology:

$$\check{H}^k(L , K) \cong H_{n-k}(M \setminus K , M \setminus L).$$

As far as this course is concerned, the most important case is when $M$ is compact. Then one takes $L = M$ and $K = \emptyset$. The left-hand side is simply the singular cohomology $H^k(M)$ (since a closed manifold is an Euclidean neighbourhood retract), and the right-hand side is the singular homology $H_{n-k}(M)$. In this case the isomorphism is realised by the cap product with the fundamental class $\langle o_M \rangle$ of $M$. This result is usually referred to as Poincaré Duality.