Maximality of orthonormal bases

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For a Hilbert space $\cal{H}$, what is the easiest way to see that an orthonormal basis $B$, for $\cal{H}$ is maximal, that is, that for any other orthonormal basis $B'$, we have $B \subseteq B'$ if and only if $B = B'$?

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Suppose $B \subset B'$, where the containment is proper. Choose $b \in B' \backslash B$; by definition of orthonormal basis, $\langle b , c \rangle = 0$ for all $c \in B$. By maximality, $b = 0$, which is a contradiction.