Definition 5.1.7 (Fundamental System of Entourages). Let $(X, \fU)$ be a uniform space, then a family $\fB \subset \fU$ is a fundamental system of entourages for $\fU$ if for every $U \in \fU$, there exists $V \in \fB$ such that $V \subset U$.
Definition 5.1.7 (Fundamental System of Entourages). Let $(X, \fU)$ be a uniform space, then a family $\fB \subset \fU$ is a fundamental system of entourages for $\fU$ if for every $U \in \fU$, there exists $V \in \fB$ such that $V \subset U$.