diff --git a/openmc/lattice.py b/openmc/lattice.py
index f0e8b40b0d2..33b0d1c738f 100644
--- a/openmc/lattice.py
+++ b/openmc/lattice.py
@@ -1301,7 +1301,7 @@ def find_element(self, point):
         else:
             z = point[2] - self.center[2]
             iz = floor(z/self.pitch[1] + 0.5*self.num_axial)
-        if self._orientation == 'x':
+        if self._orientation == 'y':
             alpha = y - x*sqrt(3.)
             i1 = floor(-alpha/(sqrt(3.0) * self.pitch[0]))
             i2 = floor(y/(sqrt(0.75) * self.pitch[0]))
@@ -1343,7 +1343,7 @@ def get_local_coordinates(self, point, idx):
             system
 
         """
-        if self._orientation == 'x':
+        if self._orientation == 'y':
             x = point[0] - (self.center[0] + (idx[0] + 0.5*idx[1])*self.pitch[0])
             y = point[1] - (self.center[1] + sqrt(0.75)*self.pitch[0]*idx[1])
         else:
@@ -1395,7 +1395,7 @@ def get_universe_index(self, idx):
             else:
                 i_within = 5*g - z
 
-        if self._orientation == 'x' and g > 0:
+        if self._orientation == 'y' and g > 0:
             i_within = (i_within + 5*g) % (6*g)
 
         if self.num_axial is None:
@@ -1556,7 +1556,7 @@ def from_xml_element(cls, elem, get_universe):
             # Get list for a single axial level
             axial_level = univs[z] if n_axial > 1 else univs
 
-            if lat.orientation == 'y':
+            if lat.orientation == 'x':
                 # Start iterating from top
                 x, alpha = 0, n_rings - 1
                 while True:
@@ -1615,7 +1615,7 @@ def _repr_axial_slice(self, universes):
         each sub-list represents a single ring.  The first list should be the
         outer ring.
         """
-        if self._orientation == 'x':
+        if self._orientation == 'y':
             return self._repr_axial_slice_x(universes)
         else:
             return self._repr_axial_slice_y(universes)
@@ -2051,7 +2051,7 @@ def show_indices(num_rings, orientation="y"):
 
         """
 
-        if orientation == 'x':
+        if orientation == 'y':
             return HexLattice._show_indices_x(num_rings)
         else:
             return HexLattice._show_indices_y(num_rings)
@@ -2095,7 +2095,7 @@ def from_hdf5(cls, group, universes):
         # If the Universe specified outer the Lattice is not void
         if outer >= 0:
             lattice.outer = universes[outer]
-        if orientation == "y":
+        if orientation == "x":
             # Build array of Universe pointers for the Lattice.  Note that
             # we need to convert between the HDF5's square array of
             # (x, alpha, z) to the Python API's format of a ragged nested
diff --git a/src/lattice.cpp b/src/lattice.cpp
index efbfcb2163a..c0068d13fab 100644
--- a/src/lattice.cpp
+++ b/src/lattice.cpp
@@ -545,7 +545,7 @@ HexLattice::HexLattice(pugi::xml_node lat_node) : Lattice {lat_node}
 
 //==============================================================================
 
-void HexLattice::fill_lattice_x(const vector<std::string>& univ_words)
+void HexLattice::fill_lattice_y(const vector<std::string>& univ_words)
 {
   int input_index = 0;
   for (int m = 0; m < n_axial_; m++) {
@@ -597,7 +597,7 @@ void HexLattice::fill_lattice_x(const vector<std::string>& univ_words)
 
 //==============================================================================
 
-void HexLattice::fill_lattice_y(const vector<std::string>& univ_words)
+void HexLattice::fill_lattice_x(const vector<std::string>& univ_words)
 {
   int input_index = 0;
   for (int m = 0; m < n_axial_; m++) {
@@ -756,7 +756,7 @@ std::pair<double, array<int, 3>> HexLattice::distance(
   double beta_dir;
   double gamma_dir;
   double delta_dir;
-  if (orientation_ == Orientation::y) {
+  if (orientation_ == Orientation::x) {
     beta_dir = u.x * std::sqrt(3.0) / 2.0 + u.y / 2.0;
     gamma_dir = u.x * std::sqrt(3.0) / 2.0 - u.y / 2.0;
     delta_dir = u.y;
@@ -785,7 +785,7 @@ std::pair<double, array<int, 3>> HexLattice::distance(
     r_t = get_local_position(r, i_xyz_t);
   }
   double beta;
-  if (orientation_ == Orientation::y) {
+  if (orientation_ == Orientation::x) {
     beta = r_t.x * std::sqrt(3.0) / 2.0 + r_t.y / 2.0;
   } else {
     beta = r_t.x;
@@ -809,7 +809,7 @@ std::pair<double, array<int, 3>> HexLattice::distance(
     r_t = get_local_position(r, i_xyz_t);
   }
   double gamma;
-  if (orientation_ == Orientation::y) {
+  if (orientation_ == Orientation::x) {
     gamma = r_t.x * std::sqrt(3.0) / 2.0 - r_t.y / 2.0;
   } else {
     gamma = r_t.x / 2.0 - r_t.y * std::sqrt(3.0) / 2.0;
@@ -836,7 +836,7 @@ std::pair<double, array<int, 3>> HexLattice::distance(
     r_t = get_local_position(r, i_xyz_t);
   }
   double delta;
-  if (orientation_ == Orientation::y) {
+  if (orientation_ == Orientation::x) {
     delta = r_t.y;
   } else {
     delta = r_t.x / 2.0 + r_t.y * std::sqrt(3.0) / 2.0;
@@ -897,7 +897,7 @@ void HexLattice::get_indices(
     }
   }
 
-  if (orientation_ == Orientation::y) {
+  if (orientation_ == Orientation::x) {
     // Convert coordinates into skewed bases.  The (x, alpha) basis is used to
     // find the index of the global coordinates to within 4 cells.
     double alpha = r_o.y - r_o.x / std::sqrt(3.0);
@@ -983,7 +983,7 @@ int HexLattice::get_flat_index(const array<int, 3>& i_xyz) const
 Position HexLattice::get_local_position(
   Position r, const array<int, 3>& i_xyz) const
 {
-  if (orientation_ == Orientation::y) {
+  if (orientation_ == Orientation::x) {
     // x_l = x_g - (center + pitch_x*cos(30)*index_x)
     r.x -=
       center_.x + std::sqrt(3.0) / 2.0 * (i_xyz[0] - n_rings_ + 1) * pitch_[0];
diff --git a/tests/regression_tests/filter_distribcell/case-4/model.xml b/tests/regression_tests/filter_distribcell/case-4/model.xml
index 8694219194b..aa5740f2f63 100644
--- a/tests/regression_tests/filter_distribcell/case-4/model.xml
+++ b/tests/regression_tests/filter_distribcell/case-4/model.xml
@@ -21,7 +21,7 @@
     <cell id="101" material="1" name="cell 2" region="-5" universe="1" />
     <cell id="102" material="2" name="cell 3" region="5" universe="1" />
     <cell id="500" material="2" name="cell 4" universe="3" />
-    <hex_lattice id="5" n_rings="2">
+    <hex_lattice id="5" n_rings="2" orientation="x">
       <pitch>1.0</pitch>
       <outer>3</outer>
       <center>0.0 0.0</center>
diff --git a/tests/regression_tests/lattice_hex/geometry.xml b/tests/regression_tests/lattice_hex/geometry.xml
index fa6a18ee16b..08666760bb4 100644
--- a/tests/regression_tests/lattice_hex/geometry.xml
+++ b/tests/regression_tests/lattice_hex/geometry.xml
@@ -43,7 +43,7 @@
 
   <!-- Lattice 101: Fuel assembly pins -->
   <cell id="100" universe="100" material="1"/>
-  <hex_lattice id="101" outer="100" n_rings="11" n_axial="3"
+  <hex_lattice id="101" outer="100" n_rings="11" orientation="x" n_axial="3"
       center="0.0 0.0 0.0" pitch="1.265 1.2">
     <universes>
                                     011
diff --git a/tests/regression_tests/lattice_hex_coincident/inputs_true.dat b/tests/regression_tests/lattice_hex_coincident/inputs_true.dat
index fafc03da1ed..a17f124b328 100644
--- a/tests/regression_tests/lattice_hex_coincident/inputs_true.dat
+++ b/tests/regression_tests/lattice_hex_coincident/inputs_true.dat
@@ -39,7 +39,7 @@
     <cell id="6" name="matrix coolant surround" material="2" region="6 2 -3" universe="2"/>
     <cell id="7" material="2" universe="3"/>
     <cell id="8" name="container cell" fill="4" region="-7 8 -9 -12 10 11 2 -3" universe="5"/>
-    <hex_lattice id="4" name="regular fuel assembly" n_rings="2">
+    <hex_lattice id="4" name="regular fuel assembly" n_rings="2" orientation="x">
       <pitch>1.4</pitch>
       <outer>3</outer>
       <center>0.0 0.0</center>
diff --git a/tests/regression_tests/lattice_hex_coincident/test.py b/tests/regression_tests/lattice_hex_coincident/test.py
index f971098c06a..b1c2ab02d4d 100644
--- a/tests/regression_tests/lattice_hex_coincident/test.py
+++ b/tests/regression_tests/lattice_hex_coincident/test.py
@@ -113,6 +113,7 @@ def __init__(self, *args, **kwargs):
         pincell_only_lattice.center = (0., 0.)
         pincell_only_lattice.pitch = (assembly_pitch,)
         pincell_only_lattice.outer = inf_mat_univ
+        pincell_only_lattice.orientation = "x"
 
         # setup hex rings
         ring0 = [fuel_ch_univ]
diff --git a/tests/regression_tests/lattice_hex_x/__init__.py b/tests/regression_tests/lattice_hex_y/__init__.py
similarity index 100%
rename from tests/regression_tests/lattice_hex_x/__init__.py
rename to tests/regression_tests/lattice_hex_y/__init__.py
diff --git a/tests/regression_tests/lattice_hex_x/inputs_true.dat b/tests/regression_tests/lattice_hex_y/inputs_true.dat
similarity index 99%
rename from tests/regression_tests/lattice_hex_x/inputs_true.dat
rename to tests/regression_tests/lattice_hex_y/inputs_true.dat
index b6536c6a8b3..708f97bde9a 100644
--- a/tests/regression_tests/lattice_hex_x/inputs_true.dat
+++ b/tests/regression_tests/lattice_hex_y/inputs_true.dat
@@ -43,7 +43,7 @@
     <cell id="11" material="2" region="8" universe="3"/>
     <cell id="12" material="2" universe="4"/>
     <cell id="13" name="container assembly cell" fill="6" region="-11 12 -13 -16 14 15 9 -10" universe="5"/>
-    <hex_lattice id="6" name="regular fuel assembly" n_rings="11" orientation="x" n_axial="2">
+    <hex_lattice id="6" name="regular fuel assembly" n_rings="11" n_axial="2">
       <pitch>1.235 5.0</pitch>
       <outer>4</outer>
       <center>0.0 0.0 5.0</center>
diff --git a/tests/regression_tests/lattice_hex_x/results_true.dat b/tests/regression_tests/lattice_hex_y/results_true.dat
similarity index 100%
rename from tests/regression_tests/lattice_hex_x/results_true.dat
rename to tests/regression_tests/lattice_hex_y/results_true.dat
diff --git a/tests/regression_tests/lattice_hex_x/test.py b/tests/regression_tests/lattice_hex_y/test.py
similarity index 99%
rename from tests/regression_tests/lattice_hex_x/test.py
rename to tests/regression_tests/lattice_hex_y/test.py
index dd5c53d0c02..51813b5ad23 100644
--- a/tests/regression_tests/lattice_hex_x/test.py
+++ b/tests/regression_tests/lattice_hex_y/test.py
@@ -166,7 +166,7 @@ def __init__(self, *args, **kwargs):
         for i, j in channels:
             universes[i][j] = abs_ch_univ
         lattice = openmc.HexLattice(lattice_id=6, name="regular fuel assembly")
-        lattice.orientation = "x"
+        lattice.orientation = "y"
         lattice.center = (0., 0., length/2.0)
         lattice.pitch = (assembly_pitch, length/2.0)
         lattice.universes = 2*[universes]
diff --git a/tests/regression_tests/lattice_rotated/inputs_true.dat b/tests/regression_tests/lattice_rotated/inputs_true.dat
index e53b93f9324..1c91e98dcfb 100644
--- a/tests/regression_tests/lattice_rotated/inputs_true.dat
+++ b/tests/regression_tests/lattice_rotated/inputs_true.dat
@@ -25,7 +25,7 @@
     <cell id="6" fill="4" region="-3" translation="-4.0 0.0 0.0" rotation="0.0 0.0 45.0" universe="5"/>
     <cell id="7" fill="3" region="-4" translation="4.0 0.0 0.0" rotation="0.0 0.0 30.0" universe="5"/>
     <cell id="8" material="3" region="-5 3 4" universe="5"/>
-    <hex_lattice id="3" n_rings="3">
+    <hex_lattice id="3" n_rings="3" orientation="x">
       <pitch>1.25</pitch>
       <outer>30</outer>
       <center>0.0 0.0</center>
diff --git a/tests/regression_tests/lattice_rotated/test.py b/tests/regression_tests/lattice_rotated/test.py
index 63641fb84a7..c2e0b505e8b 100644
--- a/tests/regression_tests/lattice_rotated/test.py
+++ b/tests/regression_tests/lattice_rotated/test.py
@@ -37,6 +37,7 @@ def rotated_lattice_model():
     pitch = 1.25
     hexlat = openmc.HexLattice()
     hexlat.center = (0., 0.)
+    hexlat.orientation = "x"
     hexlat.pitch = [pitch]
     hexlat.outer = outer_universe
     outer_ring = [big_pin_universe] + [pin_universe]*11
diff --git a/tests/unit_tests/cell_instances/test_hex_multilattice.py b/tests/unit_tests/cell_instances/test_hex_multilattice.py
index 3f503fe9b84..fa7452cbac3 100644
--- a/tests/unit_tests/cell_instances/test_hex_multilattice.py
+++ b/tests/unit_tests/cell_instances/test_hex_multilattice.py
@@ -41,7 +41,7 @@ def double_hex_lattice_model():
 
     # create a hexagonal lattice of compacts
     hex_lattice = openmc.HexLattice()
-    hex_lattice.orientation = 'y'
+    hex_lattice.orientation = 'x'
     hex_lattice.pitch = (pin_lattice_pitch,)
     hex_lattice.center = (0., 0.)
     center = [univ]
diff --git a/tests/unit_tests/test_lattice.py b/tests/unit_tests/test_lattice.py
index d72d9c5c3e6..707a0196425 100644
--- a/tests/unit_tests/test_lattice.py
+++ b/tests/unit_tests/test_lattice.py
@@ -104,6 +104,7 @@ def hlat2(pincell1, pincell2, uo2, water, zr):
     pitch = 1.2
     u1, u2 = pincell1, pincell2
     lattice = openmc.HexLattice()
+    lattice.orientation = "x"
     lattice.center = (0., 0.)
     lattice.pitch = (pitch,)
     lattice.outer = openmc.Universe(cells=[all_zr])
@@ -135,6 +136,7 @@ def hlat3(pincell1, pincell2, uo2, water, zr):
     pitch = 1.2
     u1, u2 = pincell1, pincell2
     lattice = openmc.HexLattice()
+    lattice.orientation = "x"
     lattice.center = (0., 0., 0.)
     lattice.pitch = (pitch, 10.0)
     lattice.outer = openmc.Universe(cells=[all_zr])
@@ -202,12 +204,12 @@ def test_get_universe(rlat2, rlat3, hlat2, hlat3):
     assert hlat2.get_universe((1, 0)) == u1
     assert hlat2.get_universe((-2, 2)) == u1
 
-    hlat2.orientation = 'x'
+    hlat2.orientation = 'y'
     assert hlat2.get_universe((2, 0)) == u2
     assert hlat2.get_universe((1, 0)) == u2
     assert hlat2.get_universe((1, 1)) == u1
     assert hlat2.get_universe((-1, 1)) == u1
-    hlat2.orientation = 'y'
+    hlat2.orientation = 'x'
 
     u1, u2, u3, outer = hlat3.univs
     assert hlat3.get_universe((0, 0, 0)) == u2
@@ -357,9 +359,9 @@ def test_xml_hex(hlat2, hlat3):
 
 def test_show_indices():
     for i in range(1, 11):
-        lines = openmc.HexLattice.show_indices(i).split('\n')
+        lines = openmc.HexLattice.show_indices(i, 'x').split('\n')
         assert len(lines) == 4*i - 3
-        lines_x = openmc.HexLattice.show_indices(i, 'x').split('\n')
+        lines_x = openmc.HexLattice.show_indices(i, 'y').split('\n')
         assert len(lines_x) == 4*i - 3
 
 
@@ -373,6 +375,7 @@ def test_unset_universes():
         lattice.create_xml_subelement(elem)
 
     hex_lattice = openmc.HexLattice()
+    hex_lattice.orientation = "x"
     hex_lattice.center = (0., 0.)
     hex_lattice.pitch = (1.,)
     with pytest.raises(ValueError):