diff --git a/docs/components.ipynb b/docs/components.ipynb
index bc793e5..8e250d7 100644
--- a/docs/components.ipynb
+++ b/docs/components.ipynb
@@ -135,7 +135,13 @@
     "\n",
     "Next, we add a chopper in the beamline,\n",
     "with a frequency, phase, distance from source,\n",
-    "and a set of open and close angles for the cutouts in the rotating disk."
+    "and a set of open and close angles for the cutouts in the rotating disk.\n",
+    "\n",
+    "Note that a chopper rotating anti-clockwise is given a positive frequency,\n",
+    "while a chopper rotating clockwise is given a negative frequency.\n",
+    "\n",
+    "If you define your window cutouts in the **anti-clockwise** direction,\n",
+    "your chopper should typically rotate in the **clockwise** direction so that the window with the lowest cutout angles is the first one to pass in front of the beam."
    ]
   },
   {
@@ -146,7 +152,7 @@
    "outputs": [],
    "source": [
     "chopper1 = tof.Chopper(\n",
-    "    frequency=10.0 * Hz,\n",
+    "    frequency=-10.0 * Hz,  # Negative frequency for clockwise rotation\n",
     "    open=sc.array(\n",
     "        dims=['cutout'],\n",
     "        values=[30.0, 50.0],\n",
@@ -246,7 +252,7 @@
    "outputs": [],
    "source": [
     "chopper2 = tof.Chopper(\n",
-    "    frequency=5.0 * Hz,\n",
+    "    frequency=-5.0 * Hz,\n",
     "    open=sc.array(\n",
     "        dims=['cutout'],\n",
     "        values=[30.0, 40.0, 55.0, 70.0],\n",
diff --git a/docs/multiple-pulses.ipynb b/docs/multiple-pulses.ipynb
index 003f18a..c3e5852 100644
--- a/docs/multiple-pulses.ipynb
+++ b/docs/multiple-pulses.ipynb
@@ -75,7 +75,7 @@
    "source": [
     "choppers = [\n",
     "    tof.Chopper(\n",
-    "        frequency=70.0 * Hz,\n",
+    "        frequency=-70.0 * Hz,\n",
     "        open=sc.array(\n",
     "            dims=['cutout'],\n",
     "            values=[98.71, 155.49, 208.26, 257.32, 302.91, 345.3],\n",
@@ -91,7 +91,7 @@
     "        name=\"WFM1\",\n",
     "    ),\n",
     "    tof.Chopper(\n",
-    "        frequency=70 * Hz,\n",
+    "        frequency=-70 * Hz,\n",
     "        open=sc.array(\n",
     "            dims=['cutout'],\n",
     "            values=[80.04, 141.1, 197.88, 250.67, 299.73, 345.0],\n",
@@ -107,7 +107,7 @@
     "        name=\"WFM2\",\n",
     "    ),\n",
     "    tof.Chopper(\n",
-    "        frequency=56 * Hz,\n",
+    "        frequency=-56 * Hz,\n",
     "        open=sc.array(\n",
     "            dims=['cutout'],\n",
     "            values=[74.6, 139.6, 194.3, 245.3, 294.8, 347.2],\n",
@@ -123,7 +123,7 @@
     "        name=\"Frame-overlap 1\",\n",
     "    ),\n",
     "    tof.Chopper(\n",
-    "        frequency=28 * Hz,\n",
+    "        frequency=-28 * Hz,\n",
     "        open=sc.array(\n",
     "            dims=['cutout'],\n",
     "            values=[98.0, 154.0, 206.8, 254.0, 299.0, 344.65],\n",
@@ -175,7 +175,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "res.plot(max_rays=5000)"
+    "res.plot(max_rays=10000)"
    ]
   },
   {
@@ -216,7 +216,7 @@
    "outputs": [],
    "source": [
     "pol = tof.Chopper(\n",
-    "    frequency=14 * Hz,\n",
+    "    frequency=-14 * Hz,\n",
     "    open=sc.array(\n",
     "        dims=['cutout'],\n",
     "        values=[50.0],\n",
diff --git a/docs/short-example.ipynb b/docs/short-example.ipynb
index efeeea4..542415b 100644
--- a/docs/short-example.ipynb
+++ b/docs/short-example.ipynb
@@ -77,7 +77,7 @@
    "source": [
     "choppers = [\n",
     "    tof.Chopper(\n",
-    "        frequency=70.0 * Hz,\n",
+    "        frequency=-70.0 * Hz,\n",
     "        open=sc.array(\n",
     "            dims=['cutout'],\n",
     "            values=[98.71, 155.49, 208.26, 257.32, 302.91, 345.3],\n",
@@ -93,7 +93,7 @@
     "        name=\"WFM1\",\n",
     "    ),\n",
     "    tof.Chopper(\n",
-    "        frequency=70 * Hz,\n",
+    "        frequency=-70 * Hz,\n",
     "        open=sc.array(\n",
     "            dims=['cutout'],\n",
     "            values=[80.04, 141.1, 197.88, 250.67, 299.73, 345.0],\n",
@@ -109,7 +109,7 @@
     "        name=\"WFM2\",\n",
     "    ),\n",
     "    tof.Chopper(\n",
-    "        frequency=56 * Hz,\n",
+    "        frequency=-56 * Hz,\n",
     "        open=sc.array(\n",
     "            dims=['cutout'],\n",
     "            values=[74.6, 139.6, 194.3, 245.3, 294.8, 347.2],\n",
@@ -125,7 +125,7 @@
     "        name=\"Frame-overlap 1\",\n",
     "    ),\n",
     "    tof.Chopper(\n",
-    "        frequency=28 * Hz,\n",
+    "        frequency=-28 * Hz,\n",
     "        open=sc.array(\n",
     "            dims=['cutout'],\n",
     "            values=[98.0, 154.0, 206.8, 254.0, 299.0, 344.65],\n",
@@ -141,7 +141,7 @@
     "        name=\"Frame-overlap 2\",\n",
     "    ),\n",
     "    tof.Chopper(\n",
-    "        frequency=7 * Hz,\n",
+    "        frequency=-7 * Hz,\n",
     "        open=sc.array(\n",
     "            dims=['cutout'],\n",
     "            values=[30.0],\n",
diff --git a/src/tof/chopper.py b/src/tof/chopper.py
index 403cfd0..8e81da6 100644
--- a/src/tof/chopper.py
+++ b/src/tof/chopper.py
@@ -5,6 +5,7 @@
 from dataclasses import dataclass
 from typing import Optional, Tuple
 
+import numpy as np
 import scipp as sc
 
 from .reading import ComponentReading, ReadingField
@@ -18,7 +19,8 @@ class Chopper:
     Parameters
     ----------
     frequency:
-        The frequency of the chopper.
+        The frequency of the chopper. Positive values indicate anti-clockwise rotation,
+        negative values indicate clockwise rotation.
     open:
         The opening angles of the chopper cutouts.
     close:
@@ -26,7 +28,10 @@ class Chopper:
     distance:
         The distance from the source to the chopper.
     phase:
-        The phase of the chopper.
+        The phase of the chopper. Note that the phase is applied opposite to the
+        direction of rotation. For example, if the chopper rotates clockwise, a
+        phase of 10 degrees will shift all the openings by 10 degrees in the
+        anti-clockwise direction.
     name:
         The name of the chopper.
     """
@@ -76,19 +81,43 @@ def open_close_times(
         """
         if unit is None:
             unit = time_limit.unit
-        nrot = max(int(sc.ceil((time_limit * self.frequency).to(unit='')).value), 1)
+        nrot = max(
+            int(sc.ceil((time_limit * abs(self.frequency)).to(unit='')).value), 1
+        )
         # Start at -1 to catch early openings in case the phase or opening angles are
         # large
         phases = sc.arange(uuid.uuid4().hex, -1, nrot) * two_pi + self.phase.to(
             unit='rad'
-        )
+        ) * (np.sign(self.frequency.value) * -1.0)
         # Note that the order is important here: we need (phases + open/close) to get
         # the correct dimension order when we flatten below.
-        open_times = (phases + self.open.to(unit='rad', copy=False)) / self.omega
-        close_times = (phases + self.close.to(unit='rad', copy=False)) / self.omega
+        open_times = (phases + self.open.to(unit='rad', copy=False)).flatten(
+            to=self.open.dim
+        )
+        close_times = (phases + self.close.to(unit='rad', copy=False)).flatten(
+            to=self.close.dim
+        )
+        # If the chopper is rotating anti-clockwise, we mirror the openings because the
+        # first cutout will be the last to open.
+        if self.frequency.value > 0:
+            open_times, close_times = (
+                sc.array(
+                    dims=close_times.dims,
+                    values=(two_pi - close_times).values[::-1],
+                    unit=close_times.unit,
+                ),
+                sc.array(
+                    dims=open_times.dims,
+                    values=(two_pi - open_times).values[::-1],
+                    unit=open_times.unit,
+                ),
+            )
+        abs_omg = abs(self.omega)
+        open_times /= abs_omg
+        close_times /= abs_omg
         return (
-            open_times.flatten(to=self.open.dim).to(unit=unit, copy=False),
-            close_times.flatten(to=self.close.dim).to(unit=unit, copy=False),
+            open_times.to(unit=unit, copy=False),
+            close_times.to(unit=unit, copy=False),
         )
 
     def __repr__(self) -> str:
diff --git a/tests/chopper_test.py b/tests/chopper_test.py
index e1c7a16..e277e9a 100644
--- a/tests/chopper_test.py
+++ b/tests/chopper_test.py
@@ -25,10 +25,22 @@ def test_angular_speed():
     assert chopper.omega == two_pi * f
 
 
+def test_angular_speed_negative():
+    f = -8.0 * Hz
+    chopper = tof.Chopper(
+        frequency=f,
+        open=0.0 * deg,
+        close=10.0 * deg,
+        phase=0.0 * deg,
+        distance=5.0 * meter,
+    )
+    assert chopper.omega == two_pi * f
+
+
 def test_open_close_times_one_rotation():
     f = 10.0 * Hz
     chopper = tof.Chopper(
-        frequency=f,
+        frequency=-f,  # negative frequency for clockwise rotation
         open=sc.array(dims=['cutout'], values=[10.0], unit='deg'),
         close=sc.array(dims=['cutout'], values=[20.0], unit='deg'),
         phase=0.0 * deg,
@@ -49,7 +61,7 @@ def test_open_close_times_one_rotation():
 def test_open_close_times_three_rotations():
     f = 10.0 * Hz
     chopper = tof.Chopper(
-        frequency=f,
+        frequency=-f,  # negative frequency for clockwise rotation
         open=sc.array(dims=['cutout'], values=[10.0], unit='deg'),
         close=sc.array(dims=['cutout'], values=[20.0], unit='deg'),
         phase=0.0 * deg,
@@ -79,7 +91,7 @@ def test_open_close_times_three_rotations():
 def test_open_close_angles_scalars_converted_to_arrays():
     f = 10.0 * Hz
     chopper = tof.Chopper(
-        frequency=f,
+        frequency=-f,  # negative frequency for clockwise rotation
         open=10.0 * deg,
         close=20.0 * deg,
         phase=0.0 * deg,
@@ -93,7 +105,7 @@ def test_open_close_angles_scalars_converted_to_arrays():
 def test_phase():
     f = 10.0 * Hz
     chopper1 = tof.Chopper(
-        frequency=f,
+        frequency=-f,  # negative frequency for clockwise rotation
         open=sc.array(dims=['cutout'], values=[10.0], unit='deg'),
         close=sc.array(dims=['cutout'], values=[20.0], unit='deg'),
         phase=0.0 * deg,
@@ -101,15 +113,19 @@ def test_phase():
     )
     topen1, tclose1 = chopper1.open_close_times(0.0 * sec)
     chopper2 = tof.Chopper(
-        frequency=f,
+        frequency=-f,  # negative frequency for clockwise rotation
         open=sc.array(dims=['cutout'], values=[10.0], unit='deg'),
         close=sc.array(dims=['cutout'], values=[20.0], unit='deg'),
         phase=30.0 * deg,
         distance=10.0 * meter,
     )
     topen2, tclose2 = chopper2.open_close_times(0.0 * sec)
-    assert sc.allclose(topen2, topen1 + (30.0 * deg).to(unit='rad') / chopper2.omega)
-    assert sc.allclose(tclose2, tclose1 + (30.0 * deg).to(unit='rad') / chopper2.omega)
+    assert sc.allclose(
+        topen2, topen1 + (30.0 * deg).to(unit='rad') / abs(chopper2.omega)
+    )
+    assert sc.allclose(
+        tclose2, tclose1 + (30.0 * deg).to(unit='rad') / abs(chopper2.omega)
+    )
 
 
 def test_phase_int():
@@ -118,14 +134,14 @@ def test_phase_int():
     cl = sc.array(dims=['cutout'], values=[20.0], unit='deg')
     d = 10.0 * meter
     chopper1 = tof.Chopper(
-        frequency=f,
+        frequency=-f,  # negative frequency for clockwise rotation
         open=op,
         close=cl,
         phase=30.0 * deg,
         distance=d,
     )
     chopper2 = tof.Chopper(
-        frequency=f,
+        frequency=-f,  # negative frequency for clockwise rotation
         open=op,
         close=cl,
         phase=30 * deg,
@@ -135,3 +151,60 @@ def test_phase_int():
     topen2, tclose2 = chopper2.open_close_times(0.0 * sec)
     assert sc.allclose(topen1, topen2)
     assert sc.allclose(tclose1, tclose2)
+
+
+def test_open_close_times_counter_rotation():
+    f = 10.0 * Hz
+    d = 10.0 * meter
+    ph = 0.0 * deg
+    chopper1 = tof.Chopper(
+        frequency=-f,
+        open=sc.array(dims=['cutout'], values=[10.0, 90.0], unit='deg'),
+        close=sc.array(dims=['cutout'], values=[20.0, 130.0], unit='deg'),
+        phase=ph,
+        distance=d,
+    )
+    chopper2 = tof.Chopper(
+        frequency=f,
+        open=sc.array(
+            dims=['cutout'], values=[360.0 - 130.0, 360.0 - 20.0], unit='deg'
+        ),
+        close=sc.array(
+            dims=['cutout'], values=[360.0 - 90.0, 360.0 - 10.0], unit='deg'
+        ),
+        phase=ph,
+        distance=d,
+    )
+
+    topen1, tclose1 = chopper1.open_close_times(0.2 * sec)
+    topen2, tclose2 = chopper2.open_close_times(0.0 * sec)
+    # Note that the first chopper will have one more rotation before t=0, so we slice
+    # out the first two open/close times
+    assert sc.allclose(topen1[2:], topen2)
+    assert sc.allclose(tclose1[2:], tclose2)
+
+
+def test_open_close_times_counter_rotation_with_phase():
+    f = 10.0 * Hz
+    chopper1 = tof.Chopper(
+        frequency=f,
+        open=sc.array(dims=['cutout'], values=[80.0], unit='deg'),
+        close=sc.array(dims=['cutout'], values=[90.0], unit='deg'),
+        phase=0.0 * deg,
+        distance=10.0 * meter,
+    )
+    topen1, tclose1 = chopper1.open_close_times(0.0 * sec)
+    chopper2 = tof.Chopper(
+        frequency=f,
+        open=sc.array(dims=['cutout'], values=[80.0], unit='deg'),
+        close=sc.array(dims=['cutout'], values=[90.0], unit='deg'),
+        phase=30.0 * deg,
+        distance=10.0 * meter,
+    )
+    topen2, tclose2 = chopper2.open_close_times(0.0 * sec)
+    assert sc.allclose(
+        topen2, topen1 + (30.0 * deg).to(unit='rad') / abs(chopper2.omega)
+    )
+    assert sc.allclose(
+        tclose2, tclose1 + (30.0 * deg).to(unit='rad') / abs(chopper2.omega)
+    )
diff --git a/tests/common.py b/tests/common.py
index 8c35807..ae848aa 100644
--- a/tests/common.py
+++ b/tests/common.py
@@ -13,8 +13,8 @@
 
 
 def make_chopper(topen, tclose, f, phase, distance, name):
-    aopen = two_pi * sc.concat(topen, dim='cutout').to(unit='s') * f
-    aclose = two_pi * sc.concat(tclose, dim='cutout').to(unit='s') * f
+    aopen = two_pi * sc.concat(topen, dim='cutout').to(unit='s') * (-f)
+    aclose = two_pi * sc.concat(tclose, dim='cutout').to(unit='s') * (-f)
     return tof.Chopper(
         frequency=f,
         open=aopen,
diff --git a/tests/model_test.py b/tests/model_test.py
index 11ddb8b..67a530c 100644
--- a/tests/model_test.py
+++ b/tests/model_test.py
@@ -27,7 +27,7 @@ def test_one_chopper_one_opening():
     chopper = make_chopper(
         topen=[topen],
         tclose=[tclose],
-        f=10.0 * Hz,
+        f=-10.0 * Hz,
         phase=0.0 * deg,
         distance=10 * meter,
         name='chopper',
@@ -78,7 +78,7 @@ def test_two_choppers_one_opening():
     chopper1 = make_chopper(
         topen=[topen],
         tclose=[tclose],
-        f=10.0 * Hz,
+        f=-10.0 * Hz,
         phase=0.0 * deg,
         distance=10 * meter,
         name='chopper1',
@@ -88,7 +88,7 @@ def test_two_choppers_one_opening():
     chopper2 = make_chopper(
         topen=[15.0 * ms],
         tclose=[20.0 * ms],
-        f=15.0 * Hz,
+        f=-15.0 * Hz,
         phase=0.0 * deg,
         distance=15 * meter,
         name='chopper2',
@@ -152,7 +152,7 @@ def test_two_choppers_one_and_two_openings():
     chopper1 = make_chopper(
         topen=[topen],
         tclose=[tclose],
-        f=10.0 * Hz,
+        f=-10.0 * Hz,
         phase=0.0 * deg,
         distance=10 * meter,
         name='chopper1',
@@ -161,7 +161,7 @@ def test_two_choppers_one_and_two_openings():
     chopper2 = make_chopper(
         topen=[9.0 * ms, 15.0 * ms],
         tclose=[12.0 * ms, 20.0 * ms],
-        f=15.0 * Hz,
+        f=-15.0 * Hz,
         phase=0.0 * deg,
         distance=15 * meter,
         name='chopper2',
@@ -209,7 +209,7 @@ def test_neutron_conservation():
     chopper1 = make_chopper(
         topen=[5.0 * ms],
         tclose=[16.0 * ms],
-        f=10.0 * Hz,
+        f=-10.0 * Hz,
         phase=0.0 * deg,
         distance=10 * meter,
         name='chopper1',
@@ -217,7 +217,7 @@ def test_neutron_conservation():
     chopper2 = make_chopper(
         topen=[9.0 * ms, 15.0 * ms],
         tclose=[15.0 * ms, 20.0 * ms],
-        f=15.0 * Hz,
+        f=-15.0 * Hz,
         phase=0.0 * deg,
         distance=15 * meter,
         name='chopper2',
@@ -337,7 +337,7 @@ def test_model_repr_does_not_raise():
     chopper1 = make_chopper(
         topen=[5.0 * ms],
         tclose=[16.0 * ms],
-        f=10.0 * Hz,
+        f=-10.0 * Hz,
         phase=0.0 * deg,
         distance=10 * meter,
         name='chopper1',
@@ -345,7 +345,7 @@ def test_model_repr_does_not_raise():
     chopper2 = make_chopper(
         topen=[9.0 * ms, 15.0 * ms],
         tclose=[15.0 * ms, 20.0 * ms],
-        f=15.0 * Hz,
+        f=-15.0 * Hz,
         phase=0.0 * deg,
         distance=15 * meter,
         name='chopper2',
diff --git a/tests/result_test.py b/tests/result_test.py
index ef01b33..d09f54e 100644
--- a/tests/result_test.py
+++ b/tests/result_test.py
@@ -23,7 +23,7 @@ def chopper():
     return make_chopper(
         topen=[topen],
         tclose=[tclose],
-        f=10.0 * Hz,
+        f=-10.0 * Hz,
         phase=0.0 * deg,
         distance=10 * meter,
         name='chopper',