diff --git a/Frechet_distance/doc/Frechet_distance/Concepts/FrechetDistanceTraits.h b/Frechet_distance/doc/Frechet_distance/Concepts/FrechetDistanceTraits.h
index 111a0890889..4724f6e4846 100644
--- a/Frechet_distance/doc/Frechet_distance/Concepts/FrechetDistanceTraits.h
+++ b/Frechet_distance/doc/Frechet_distance/Concepts/FrechetDistanceTraits.h
@@ -30,7 +30,7 @@ Point type.
 using Point_d = unspecified_type;
 
 /*!
-The number type of the %Cartesian coordinates of type `Point_d`.
+The number type of the %Cartesian coordinates of type `Point_d`.  It must be a model of `FieldNumberType`.
 For a given `FT n`, `to_interval(n)` must be a valid expression and it must
 return an interval containing `n`, represented by a `std::pair<double, double>`.
 */