diff --git a/reticule.py b/reticule.py
new file mode 100644
index 0000000..fa8c424
--- /dev/null
+++ b/reticule.py
@@ -0,0 +1,96 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+#
+#    Copyright © 2024 Simon Forman
+#
+#    This file is part of game
+#
+#    game is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+#
+#    game is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU General Public License for more details.
+#
+#    You should have received a copy of the GNU General Public License
+#    along with game.  If not see <http://www.gnu.org/licenses/>.
+#
+
+
+class Reticule:
+
+    def __init__(self, canvas, radius_x=35, radius_y=20, color='green'):
+        self.canvas = canvas
+        self.radius_x = radius_x
+        self.radius_y = radius_y
+        self.color = color
+        self.item_ids = []
+        self.init_reticule()
+
+    def get_reticule_coords(self, x, y):
+        '''
+        Given the width and height of a rectangle and x-y coords in that
+        rectangle yield coords for four lines and an oval suitable for
+        passing to canvas.coords() method.
+        '''
+        width, height = int(self.canvas['width']), int(self.canvas['height'])
+        top = y - self.radius_y
+        left = x - self.radius_x
+        bottom = y + self.radius_y
+        right = x + self.radius_x
+        yield 0, y, left, y
+        yield right, y, width - 1, y
+        yield x, 0, x, top
+        yield x, bottom, x, height - 1
+        yield left, top, right, bottom
+
+    def init_reticule(self):
+        append = self.item_ids.append
+        w, h = int(self.canvas['width']), int(self.canvas['height'])
+        coords = self.get_reticule_coords(w >> 1, h >> 1)  # Center.
+        append(canvas.create_line(*next(coords), fill=self.color))
+        append(canvas.create_line(*next(coords), fill=self.color))
+        append(canvas.create_line(*next(coords), fill=self.color))
+        append(canvas.create_line(*next(coords), fill=self.color))
+        append(canvas.create_oval(*next(coords), outline=self.color))
+
+    def set_reticule(self, x, y):
+        coords = self.get_reticule_coords(x, y)
+        for item_id, item_coords in zip(self.item_ids, coords):
+            self.canvas.coords(item_id, *item_coords)
+
+    def handle_set_event(self, event):
+        self.set_reticule(event.x, event.y)
+
+
+if __name__ == '__main__':
+    from tkinter import *
+
+    def handle_canvas_resize(event):
+        # I don't know why the Tk system doesn't update these values.
+        canvas['width'] = event.width
+        canvas['height'] = event.height
+        # update reticule here/now?
+        # If the canvas window has enlarged then some of the lines will have
+        # to be extended.
+
+
+    canvas = Canvas(bg='black')
+
+    canvas.bind('<Configure>', handle_canvas_resize)
+    canvas.pack(expand=True, fill=BOTH)
+    canvas.update()
+
+    # Let the configure event propagate and trigger the
+    # handle_canvas_resize() callback to set the width
+    # and height values of the canvas correctly so that
+    # the reticule lines get drawn correctly.
+
+    reticule = Reticule(canvas)
+    canvas.bind('<Button-1>', reticule.handle_set_event)
+    canvas.bind('<B1-Motion>', reticule.handle_set_event)
+
+    #canvas.mainloop()