Definition 17.1.1 (Compatible Couple).label Let $E_{0}, E_{1}, \mathcal{U}$ be topological vector spaces over $K \in \RC$ and $\iota_{0} \in L(E_{0}; \mathcal{U})$ and $\iota_{1} \in L(E_{1}; \mathcal{U})$ be continuous injections. Under the identification that $E_{0}$ and $E_{1}$ are subspaces of $\mathcal{U}$, the pair $(E_{0}, E_{1})$ forms a compatible couple of topological vector spaces.