diff --git a/Lib/fontTools/pens/t2CharStringPen.py b/Lib/fontTools/pens/t2CharStringPen.py
index d9f8498..d8e0338 100644
--- a/Lib/fontTools/pens/t2CharStringPen.py
+++ b/Lib/fontTools/pens/t2CharStringPen.py
@@ -91,6 +91,15 @@
 
 
 class T2CharStringPen(RelativeCoordinatePen):
+    """Pen to draw Type 2 CharStrings.
+
+    The 'roundTolerance' argument controls the rounding of point coordinates.
+    It is defined as the maximum absolute difference between the original
+    float and the rounded integer value.
+    The default tolerance of 0.5 means that all floats are rounded to integer;
+    a value of 0 disables rounding; values in between will only round floats
+    which are close to their integral part within the tolerated range.
+    """
 
     def __init__(self, width, glyphSet, roundTolerance=0.5):
         RelativeCoordinatePen.__init__(self, glyphSet)