\begin{definition}[Definition:Euclid's Definitions - Book I/21 - Triangles by Angles]
{{EuclidSaid}}
:''Further, of trilateral figures, a '''right-angled triangle''' is that which has a right angle, an '''obtuse-angled triangle''' that which has an obtuse angle, and an '''acute-angled triangle''' that which has its three angles acute.''
{{EuclidDefRef|I|21|Triangles by Angles}}
\end{definition}