Let $X$ be a smooth manifold, let $P \in \Psi_{\mathrm{cl}}^m(X)$ be a properly supported classical pseudodifferential operator of order $m$ whose real principal symbol $p$ is of real principal type on $\operatorname{Char}(P) = \{(x,\xi) \in T^*X \setminus 0 : p(x,\xi)=0\}$, and let $u \in \mathcal{D}'(X)$. Let $\gamma: [a,b] \to \operatorname{Char}(P)$ be a bicharacteristic segment for the Hamilton vector field $H_p$, with $a < b$. Assume

\begin{align*} \gamma([a,b]) \cap WF(Pu) = \varnothing. \end{align*}

If $\gamma(a) \notin WF(u)$, then

\begin{align*} \gamma([a,b]) \cap WF(u) = \varnothing. \end{align*}

The same conclusion holds with $a$ and $b$ interchanged: if $\gamma(b) \notin WF(u)$, then $\gamma([a,b]) \cap WF(u) = \varnothing$.