diff --git a/prjxray/overlay.py b/prjxray/overlay.py
new file mode 100644
index 00000000..64c7cce6
--- /dev/null
+++ b/prjxray/overlay.py
@@ -0,0 +1,29 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+#
+# Copyright (C) 2017-2020  The Project X-Ray Authors.
+#
+# Use of this source code is governed by a ISC-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/ISC
+#
+# SPDX-License-Identifier: ISC
+class Overlay(object):
+    """ Object that represents an overlay.
+
+    Can be used to iterate over tiles and sites not inside a partition region.
+
+    """
+
+    def __init__(self, region_dict):
+        self.region_dict = region_dict
+
+    def tile_in_roi(self, grid_loc):
+        """ Returns true if grid_loc (GridLoc tuple) is within the overlay. """
+        x = grid_loc.grid_x
+        y = grid_loc.grid_y
+        for _, bounds in self.region_dict.items():
+            x1, x2, y1, y2 = bounds
+            if x1 <= x and x <= x2 and y1 <= y and y <= y2:
+                return False
+        return True
diff --git a/tests/test_util.py b/tests/test_util.py
index 8ea851ed..2187630c 100755
--- a/tests/test_util.py
+++ b/tests/test_util.py
@@ -21,7 +21,9 @@ from unittest import TestCase, main
 environ['XRAY_DATABASE_ROOT'] = '.'
 environ['XRAY_PART'] = './'
 
-from prjxray.util import get_roi
+from prjxray.util import get_roi, get_db_root
+from prjxray.overlay import Overlay
+from prjxray.grid_types import GridLoc
 
 
 @contextmanager
@@ -69,6 +71,17 @@ class TestUtil(TestCase):
             self.assertListEqual(
                 list(get_roi().gen_sites()), [('ATILE', 'FOO', 'BAR')])
 
+    def test_in_roi_overlay(self):
+        region_dict = {}
+        region_dict['pr1'] = (10, 58, 0, 51)
+        region_dict['pr2'] = (10, 58, 52, 103)
+        overlay = Overlay(region_dict)
+        self.assertFalse(overlay.tile_in_roi(GridLoc(18, 50)))
+        self.assertFalse(overlay.tile_in_roi(GridLoc(18, 84)))
+        self.assertTrue(overlay.tile_in_roi(GridLoc(8, 50)))
+        self.assertTrue(overlay.tile_in_roi(GridLoc(18, 112)))
+        self.assertTrue(overlay.tile_in_roi(GridLoc(80, 40)))
+
 
 if __name__ == '__main__':
     main()