diff --git a/CHANGELOG b/CHANGELOG
index 7fa5ba1..6f82d60 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -5,6 +5,7 @@
 	* Dynamic bits
 	* Optional binary contents
 	* Sync all if no state passed as argument
+	* Mask method for client-side state manipulation
 - 0.1.0
 	* Release version bump
 - 0.0.19:
diff --git a/shep/state.py b/shep/state.py
index c4bbaf2..0ff02df 100644
--- a/shep/state.py
+++ b/shep/state.py
@@ -92,10 +92,11 @@ class State:
 
 
     # enforces state value within bit limit of instantiation
-    def __check_limit(self, v):
-        if self.__initial_bits == 0:
-            self.__bits += 1
-        self.__limit = (1 << self.__bits) - 1
+    def __check_limit(self, v, pure=True):
+        if pure:
+            if self.__initial_bits == 0:
+                self.__bits += 1
+            self.__limit = (1 << self.__bits) - 1
         if v > self.__limit:
             raise OverflowError(v)
         return v
@@ -197,7 +198,7 @@ class State:
         v = 0
         for a in args:
             a = self.__check_value_cursor(a)
-            v = self.__check_limit(v | a)
+            v = self.__check_limit(v | a, pure=False)
         if self.__is_pure(v):
             raise ValueError('use add to add pure values')
         self.__set(k, v)
@@ -593,3 +594,11 @@ class State:
 
     def register_modify(self, key):
         self.modified_last[key] = datetime.datetime.now().timestamp()
+
+
+    def mask(self, key, states):
+        statemask = self.__limit + 1
+        statemask |= states
+        statemask = ~statemask
+        statemask &= self.__limit
+        return statemask
diff --git a/tests/test_state.py b/tests/test_state.py
index cf24c37..eefe6ff 100644
--- a/tests/test_state.py
+++ b/tests/test_state.py
@@ -216,5 +216,26 @@ class TestState(unittest.TestCase):
         states.alias('baz', states.FOO | states.BAR)
 
 
+
+    def test_mask(self):
+        states = State(3)
+        states.add('foo')
+        states.add('bar')
+        states.add('baz')
+        states.alias('all', states.FOO | states.BAR | states.BAZ)
+        mask = states.mask('xyzzy', states.FOO | states.BAZ)
+        self.assertEqual(mask, states.BAR)
+
+
+    def test_mask_dynamic(self):
+        states = State(0)
+        states.add('foo')
+        states.add('bar')
+        states.add('baz')
+        states.alias('all', states.FOO | states.BAR | states.BAZ)
+        mask = states.mask('xyzzy', states.FOO | states.BAZ)
+        self.assertEqual(mask, states.BAR)
+
+
 if __name__ == '__main__':
     unittest.main()