\begin{definition}[Definition:Angle/Rectilineal]
{{EuclidSaid}}
:''{{:Definition:Euclid's Definitions - Book I/9 - Rectilineal Angle}}''
{{EuclidDefRefNocat|I|9|Rectilineal Angle}}
Thus the distinction is made between straight-line angles and curved-line angles.
Most of the time the fact that angles are rectilineal is taken for granted.
\end{definition}