diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
index 5e09dde41b23..903d62fb5c00 100644
--- a/lib/matplotlib/ticker.py
+++ b/lib/matplotlib/ticker.py
@@ -326,6 +326,9 @@ def set_offset_string(self, ofs):
 class FuncFormatter(Formatter):
     """
     User defined function for formatting
+
+    The function should take in two inputs (tick value *x* and position *pos*)
+    and return a string
     """
     def __init__(self, func):
         self.func = func