diff --git a/face.cpp b/face.cpp
index 8b81939..9dae758 100644
--- a/face.cpp
+++ b/face.cpp
@@ -7,6 +7,7 @@
 #include <iostream>
 #include <stdexcept>
 #include <limits>
+
 #include "face.h"
 #include "edge.h"
 
@@ -187,4 +188,28 @@ double face::calc_area() const
 	return(0.5*(A|B).length());
 }
 
+/*!
+*	Checks wether the face is an obtuse triangle. This requires evaluating
+*	the `is_obtuse` flag. If the value of this flag has not yet been set,
+*	the function performs an obtusity test, sets the flag, and returns its
+*	value.
+*
+*	@return true if the face is an obtuse triangle, else false
+*/
+
+bool face::is_obtuse()
+{
+	using boost::logic::tribool;
+	if(indeterminate(obtuse))
+	{
+		double a = E[0].e->calc_length();
+		double b = E[1].e->calc_length();
+		double c = E[2].e->calc_length();
+
+		obtuse = (a*a+b*b < c*c) || (b*b* + c*c < a*a) || (c*c + a*a < b*b);
+	}
+
+	return(obtuse == true);
+}
+
 } // end of namespace "psalm"
diff --git a/face.h b/face.h
index daaf067..8248968 100644
--- a/face.h
+++ b/face.h
@@ -6,6 +6,8 @@
 #ifndef __FACE_H__
 #define __FACE_H__
 
+#include <boost/logic/tribool.hpp>
+
 #include "vertex.h"
 #include "directed_edge.h"
 
@@ -50,6 +52,8 @@ class face
 		bool is_on_boundary() const;
 		void set_on_boundary(bool boundary = true);
 
+		bool is_obtuse();
+
 		void reconstruct_from_edges();
 
 		double calc_area() const;
@@ -64,6 +68,16 @@ class face
 
 		bool boundary;	///< Flag signalling that the face is a
 				///< boundary face.
+
+		/*!
+		*	Flag signalling that the face is an obtuse triangle.
+		*	This makes only sense for triangles, of course. By
+		*	using the tribool, the function `is_obtuse()` may
+		*	decide whether it is required to _calculate_ the value
+		*	of the flag or can simply return it.
+		*/
+
+		boost::logic::tribool obtuse;
 };
 
 } // end of namespace "psalm"