Euler angles describe an aircraft's orientation relative to
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The three Euler angles are
\[
\boxed{
\begin{aligned}
\phi &:& \text{Roll}
\theta &:& \text{Pitch}
\psi &:& \text{Yaw}
\end{aligned}
}
\]
They describe the orientation of the aircraft body axes with respect to the inertial (Earth-fixed) reference frame.