diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
index f6729dc7e38..6657fd41072 100644
--- a/lib/matplotlib/axes/_base.py
+++ b/lib/matplotlib/axes/_base.py
@@ -2470,7 +2470,8 @@ def grid(self, b=None, which='major', axis='both', **kwargs):
         """
         if len(kwargs):
             b = True
-        b = _string_to_bool(b)
+        elif b is not None:
+            b = _string_to_bool(b)
 
         if axis == 'x' or axis == 'both':
             self.xaxis.grid(b, which=which, **kwargs)
diff --git a/lib/matplotlib/axis.py b/lib/matplotlib/axis.py
index 7f10f0407be..dc3f0bb4332 100644
--- a/lib/matplotlib/axis.py
+++ b/lib/matplotlib/axis.py
@@ -10,6 +10,7 @@
 import matplotlib.artist as artist
 from matplotlib.artist import allow_rasterization
 import matplotlib.cbook as cbook
+from matplotlib.cbook import _string_to_bool
 import matplotlib.font_manager as font_manager
 import matplotlib.lines as mlines
 import matplotlib.patches as mpatches
@@ -802,17 +803,6 @@ def set_tick_params(self, which='major', reset=False, **kw):
 
     @staticmethod
     def _translate_tick_kw(kw, to_init_kw=True):
-        # We may want to move the following function to
-        # a more visible location; or maybe there already
-        # is something like this.
-        def _bool(arg):
-            if cbook.is_string_like(arg):
-                if arg.lower() == 'on':
-                    return True
-                if arg.lower() == 'off':
-                    return False
-                raise ValueError('String "%s" should be "on" or "off"' % arg)
-            return bool(arg)
         # The following lists may be moved to a more
         # accessible location.
         kwkeys0 = ['size', 'width', 'color', 'tickdir', 'pad',
@@ -828,22 +818,22 @@ def _bool(arg):
             if 'direction' in kw:
                 kwtrans['tickdir'] = kw.pop('direction')
             if 'left' in kw:
-                kwtrans['tick1On'] = _bool(kw.pop('left'))
+                kwtrans['tick1On'] = _string_to_bool(kw.pop('left'))
             if 'bottom' in kw:
-                kwtrans['tick1On'] = _bool(kw.pop('bottom'))
+                kwtrans['tick1On'] = _string_to_bool(kw.pop('bottom'))
             if 'right' in kw:
-                kwtrans['tick2On'] = _bool(kw.pop('right'))
+                kwtrans['tick2On'] = _string_to_bool(kw.pop('right'))
             if 'top' in kw:
-                kwtrans['tick2On'] = _bool(kw.pop('top'))
+                kwtrans['tick2On'] = _string_to_bool(kw.pop('top'))
 
             if 'labelleft' in kw:
-                kwtrans['label1On'] = _bool(kw.pop('labelleft'))
+                kwtrans['label1On'] = _string_to_bool(kw.pop('labelleft'))
             if 'labelbottom' in kw:
-                kwtrans['label1On'] = _bool(kw.pop('labelbottom'))
+                kwtrans['label1On'] = _string_to_bool(kw.pop('labelbottom'))
             if 'labelright' in kw:
-                kwtrans['label2On'] = _bool(kw.pop('labelright'))
+                kwtrans['label2On'] = _string_to_bool(kw.pop('labelright'))
             if 'labeltop' in kw:
-                kwtrans['label2On'] = _bool(kw.pop('labeltop'))
+                kwtrans['label2On'] = _string_to_bool(kw.pop('labeltop'))
             if 'colors' in kw:
                 c = kw.pop('colors')
                 kwtrans['color'] = c
diff --git a/lib/matplotlib/cbook.py b/lib/matplotlib/cbook.py
index 5e2de452c50..f7ba4d95ca4 100644
--- a/lib/matplotlib/cbook.py
+++ b/lib/matplotlib/cbook.py
@@ -785,13 +785,15 @@ def is_scalar_or_string(val):
 
 
 def _string_to_bool(s):
+    """Parses the string argument as a boolean"""
     if not is_string_like(s):
-        return s
-    if s == 'on':
+        return bool(s)
+    if s.lower() in ['on', 'true']:
         return True
-    if s == 'off':
+    if s.lower() in ['off', 'false']:
         return False
-    raise ValueError("string argument must be either 'on' or 'off'")
+    raise ValueError('String "%s" must be one of: '
+                     '"on", "off", "true", or "false"' % s)
 
 
 def get_sample_data(fname, asfileobj=True):
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
index 039382cc4a5..a1a69342ede 100644
--- a/lib/matplotlib/pyplot.py
+++ b/lib/matplotlib/pyplot.py
@@ -29,8 +29,8 @@
 import matplotlib.colorbar
 from matplotlib import style
 from matplotlib import _pylab_helpers, interactive
-from matplotlib.cbook import dedent, silent_list, is_string_like, is_numlike
-from matplotlib.cbook import _string_to_bool
+from matplotlib.cbook import (dedent, silent_list, is_string_like, is_numlike,
+                              _string_to_bool)
 from matplotlib import docstring
 from matplotlib.backend_bases import FigureCanvasBase
 from matplotlib.figure import Figure, figaspect