An easy application of the Yoneda lemma completely characterizes natural transformations between representable functors: any locally small category $\mathsf{C}$ is isomorphic to the full subcategory of $\textup{\textsf{cat}}^{\mathsf{C}^\mathrm{op}}$ spanned by the contravariant represented functors, and $\mathsf{C}^\mathrm{op}$ is isomorphic to the full subcategory of $\textup{\textsf{cat}}^{\mathsf{C}}$ spanned by the covariant represented functors, via the canonically-defined covariant and contravariant Yoneda embeddings.