diff --git a/segeval/similarity/__init__.py b/segeval/similarity/__init__.py
index 220811b..b780e6e 100644
--- a/segeval/similarity/__init__.py
+++ b/segeval/similarity/__init__.py
@@ -113,11 +113,11 @@ def __boundary_confusion_matrix__(*args, **kwargs):
         hyp, ref = None, None
         boundary_type, side = addition
         if side == 'a':
-            hyp = None
-            ref = boundary_type
-        else:  # side == 'b'
             hyp = boundary_type
             ref = None
+        else:  # side == 'b'
+            hyp = None
+            ref = boundary_type
         assert side == 'a' or side == 'b'
         matrix[hyp][ref] += 1
     return matrix
diff --git a/segeval/similarity/test.py b/segeval/similarity/test.py
index 0042198..13480c2 100644
--- a/segeval/similarity/test.py
+++ b/segeval/similarity/test.py
@@ -23,18 +23,17 @@ def test_boundary_confusion_matrix(self):
         Test confusion matrix.
         '''
         cm = boundary_confusion_matrix(
-            [set([]), set([2]), set([]), set([]), set([1]), set([1]),
-             set([1]), set([1])],
-            [set([1]), set([1]), set([]), set([1]), set([]), set([1]),
-             set([]), set([])],
+            #    A-b        S         M         T --------T         M        A-a       A-a
+            [set([ ]), set([2]), set([ ]), set([ ]), set([1]), set([1]), set([1]), set([1])],
+            [set([1]), set([1]), set([ ]), set([1]), set([ ]), set([1]), set([ ]), set([ ])],
             boundary_format=BoundaryFormat.sets)
-        self.assertEqual(cm[None][1], 2)
-        self.assertEqual(cm[1][None], 1)
+        self.assertEqual(cm[None][1], 1)           # A-b
+        self.assertEqual(cm[1][None], 2)           # A-a
         self.assertEqual(cm[None][2], 0)
         self.assertEqual(cm[2][None], 0)
-        self.assertEqual(cm[2][1], 1)
+        self.assertEqual(cm[2][1], 1)              # S
         self.assertEqual(cm[1][2], 0)
-        self.assertEqual(cm[1][1], Decimal('1.5'))
+        self.assertEqual(cm[1][1], Decimal('1.5')) # M + T
         self.assertEqual(cm[2][2], 0)
 
     def test_boundary_statistics(self):