diff --git a/poincare/include/poincare/expression.h b/poincare/include/poincare/expression.h
index a89180c21..912bdf1a4 100644
--- a/poincare/include/poincare/expression.h
+++ b/poincare/include/poincare/expression.h
@@ -116,6 +116,10 @@ public:
   /* Sorting */
   virtual bool isCommutative() const { return false; }
   virtual void sort();
+  /* compareTo returns:
+   *   1 if this > e
+   *   -1 if this < e
+   *   0 if this == e */
   virtual int compareTo(const Expression * e) const;
 
   /* Layout Engine */
diff --git a/poincare/src/expression.cpp b/poincare/src/expression.cpp
index 2acda2905..d18ab884d 100644
--- a/poincare/src/expression.cpp
+++ b/poincare/src/expression.cpp
@@ -97,12 +97,12 @@ void Expression::sort() {
 }
 
 int Expression::compareTo(const Expression * e) const {
-  if (e->type() > this->type()) {
-    return 1;
-  }
-  if (e->type() < this->type()) {
+  if (this->type() < e->type()) {
     return -1;
   }
+  if (this->type() > e->type()) {
+    return 1;
+  }
   for (int i = 0; i < this->numberOfOperands(); i++) {
     // The NULL node is the least node type.
     if (e->numberOfOperands() <= i) {