diff --git a/poincare/include/poincare/trigonometry.h b/poincare/include/poincare/trigonometry.h
index 8c038e24e..424f1e80f 100644
--- a/poincare/include/poincare/trigonometry.h
+++ b/poincare/include/poincare/trigonometry.h
@@ -16,6 +16,8 @@ public:
   static bool isDirectTrigonometryFunction(const Expression & e);
   static bool isInverseTrigonometryFunction(const Expression & e);
   static bool AreInverseFunctions(const Expression & directFunction, const Expression & inverseFunction);
+  /* Returns a (unreduced) division between pi in each unit, or 1 if the units
+   * are the same. */
   static Expression UnitConversionFactor(Preferences::AngleUnit fromUnit, Preferences::AngleUnit toUnit);
   static bool ExpressionIsEquivalentToTangent(const Expression & e);
   static Expression shallowReduceDirectFunction(Expression & e, ExpressionNode::ReductionContext reductionContext);
diff --git a/poincare/src/trigonometry.cpp b/poincare/src/trigonometry.cpp
index a9c849c33..bdaa0a165 100644
--- a/poincare/src/trigonometry.cpp
+++ b/poincare/src/trigonometry.cpp
@@ -90,6 +90,7 @@ bool Trigonometry::AreInverseFunctions(const Expression & directFunction, const
 
 Expression Trigonometry::UnitConversionFactor(Preferences::AngleUnit fromUnit, Preferences::AngleUnit toUnit) {
   if (fromUnit == toUnit) {
+    // Just an optimisation to gain some time at reduction
     return Rational::Builder(1);
   }
   return Division::Builder(piExpression(toUnit), piExpression(fromUnit));