diff --git a/pandas/util/testing.py b/pandas/util/testing.py
index 9659cb33686d0..0278432117631 100644
--- a/pandas/util/testing.py
+++ b/pandas/util/testing.py
@@ -990,6 +990,12 @@ def assert_series_equal(left, right, check_dtype=True,
         Specify comparison precision. Only used when check_exact is False.
         5 digits (False) or 3 digits (True) after decimal points are compared.
         If int, then specify the digits to compare.
+
+        When comparing two numbers, if the first number has magnitude less
+        than 1e-5, we compare the two numbers directly and check whether
+        they are equivalent within the specified precision. Otherwise, we
+        compare the **ratio** of the second number to the first number and
+        check whether it is equivalent to 1 within the specified precision.
     check_names : bool, default True
         Whether to check the Series and Index names attribute.
     check_exact : bool, default False
@@ -1131,6 +1137,12 @@ def assert_frame_equal(left, right, check_dtype=True,
         Specify comparison precision. Only used when check_exact is False.
         5 digits (False) or 3 digits (True) after decimal points are compared.
         If int, then specify the digits to compare.
+
+        When comparing two numbers, if the first number has magnitude less
+        than 1e-5, we compare the two numbers directly and check whether
+        they are equivalent within the specified precision. Otherwise, we
+        compare the **ratio** of the second number to the first number and
+        check whether it is equivalent to 1 within the specified precision.
     check_names : bool, default True
         Whether to check that the `names` attribute for both the `index`
         and `column` attributes of the DataFrame is identical, i.e.