diff --git a/setclass/__init__.py b/setclass/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/setclass.py b/setclass/setclass.py
similarity index 71%
rename from setclass.py
rename to setclass/setclass.py
index 4449623..1ce37b9 100644
--- a/setclass.py
+++ b/setclass/setclass.py
@@ -168,72 +168,3 @@ class SetClass(list):
             pitches += f"{pitch}{sub}"
         sup = superscript(self.brightness)
         return f"[{pitches}]{sup}"
-
-
-if __name__ == "__main__":
-    print("Running tests:")
-
-    # Forte 0-1, the empty set
-    empty = SetClass()
-    print(f"Forte 0-1: {empty}")
-    assert empty.tonality == 12
-    assert empty.cardinality == 0
-    assert empty.brightness == 0
-    assert len(empty) == 0
-    assert len(empty.pitch_classes) == 0
-    assert len(empty.adjacency_intervals) == 0
-    assert empty.leonard_notation == '[]⁰'
-    print(f"  Leonard: {empty.leonard_notation}")
-    assert len(empty.versions) == 1
-    print(f"  Versions ({len(empty.versions)}):")
-    for i in empty.versions:
-        print(f"    {i.leonard_notation}")
-    assert empty.brightest_form == empty.darkest_form
-
-    # Forte 12-1, the complement of empty (full set)
-    f121 = empty.complement
-    print(f"Forte 12-1: {empty}")
-    assert f121 == SetClass(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
-    assert f121.tonality == 12
-    assert f121.cardinality == 12
-    assert f121.brightness == 66
-    assert len(f121) == 12
-    assert len(f121.pitch_classes) == 12
-    assert len(f121.adjacency_intervals) == 12
-    assert f121.leonard_notation == '[0₁1₁2₁3₁4₁5₁6₁7₁8₁9₁10₁11₁]⁶⁶'
-    assert len(f121.versions) == 1
-    assert f121.brightest_form == f121.darkest_form
-
-    # Forte 5-20
-    f520 = SetClass(0, 1, 5, 6, 8)
-    print(f"{f520}")
-    assert f520.tonality == 12
-    assert f520.cardinality == 5
-    assert f520.brightness == 20
-    assert len(f520) == 5
-    assert len(f520.pitch_classes) == 5
-    print(f"Adjacency intervals: {f520.adjacency_intervals}")
-    assert len(f520.adjacency_intervals) == 5
-    assert len(f520.versions) == 5
-    print(f"Leonard notation: {f520.leonard_notation}")
-    assert f520.leonard_notation == '[0₁1₄5₁6₂8₄]²⁰'
-
-    assert f520.brightest_form.leonard_notation == '[0₄4₁5₄9₁10₂]²⁸'
-    assert f520.brightest_form == SetClass(0, 4, 5, 9, 10)
-
-    sc = f520
-    co = f520.complement
-    print(f"Versions ({len(sc.versions)}):")
-    for i in sc.versions:
-        print(f"    {i.leonard_notation}")
-    print(f"Brightest form: {sc.brightest_form.leonard_notation}")
-    print(f"Darkest form: {sc.darkest_form.leonard_notation}")
-    print()
-
-    print(f"Complement: {co}")
-    print(f"Adjacency intervals: {co.adjacency_intervals}")
-    print(f"Leonard notation: {co.leonard_notation}")
-    print(f"Versions ({len(co.versions)}):")
-    for i in co.versions:
-        print(f"    {i.leonard_notation}")
-    print(f"Darkest form: {co.darkest_form}")
diff --git a/setclass/tests/__init__.py b/setclass/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/setclass/tests/test_setclass.py b/setclass/tests/test_setclass.py
new file mode 100644
index 0000000..cf8e158
--- /dev/null
+++ b/setclass/tests/test_setclass.py
@@ -0,0 +1,250 @@
+from setclass.setclass import SetClass
+
+
+# ----------------------------------------------------------------------------
+# Functional tests
+
+
+def test_zero_normalised_intervals():
+    a = SetClass(0, 1, 3)
+    assert a == SetClass(1, 2, 4)
+    assert a == SetClass(4, 5, 7)
+
+
+def test_modulo_intervals():
+    a = SetClass(0, 1, 3)
+    assert a == SetClass(12, 13, 15)  # modulo down
+    assert a == SetClass(-11, -9, -12)  # modulo up
+
+
+def test_negative_intervals():
+    a = SetClass(0, 9, 10)
+    assert a == SetClass(-3, -2, 0)
+
+
+def test_immutable_intervals():
+    a = SetClass(0, 1, 3)
+    assert a == SetClass(0, 3, 1)
+    assert a == SetClass(3, 0, 1)
+    assert a == SetClass(1, 3, 0)
+    assert a == SetClass(1, 0, 3)
+    assert a == SetClass(3, 1, 0)
+
+
+def test_repeat_intervals():
+    a = SetClass(0, 1, 3)
+    assert a == SetClass(0, 1, 3, 3, 1)
+    assert a == SetClass(0, 12, 1, -11, 15)
+    assert a == SetClass(12, 1, -11, 15, -9)
+
+
+# ----------------------------------------------------------------------------
+# Example Forte 5-20 set class
+f520 = SetClass(0, 1, 5, 6, 8)
+
+
+def test_tonality():
+    assert f520.tonality == 12
+
+
+def test_cardinality():
+    assert f520.cardinality == 5
+
+
+def test_brightness():
+    assert f520.brightness == 20
+
+
+def test_pitch_classes():
+    assert len(f520) == 5
+    assert len(f520.pitch_classes) == 5
+
+
+def test_adjacency_intervals():
+    assert len(f520.adjacency_intervals) == 5
+
+
+def test_versions():
+    assert len(f520.versions) == 5
+
+
+def test_brightest_form():
+    b = f520.brightest_form
+    assert b in f520.versions
+    assert b == SetClass(0, 4, 5, 9, 10)
+    assert b.leonard_notation == '[0₄4₁5₄9₁10₂]²⁸'
+
+
+def test_darkest_form():
+    d = f520.darkest_form
+    assert d in f520.versions
+    assert d == SetClass(0, 1, 3, 7, 8)
+    assert d.leonard_notation == '[0₁1₂3₄7₁8₄]¹⁹'
+
+
+def test_leonard_notation():
+    assert f520.leonard_notation == '[0₁1₄5₁6₂8₄]²⁰'
+
+
+# ----------------------------------------------------------------------------
+# Complement of Forte 5-20 set class: Forte 7-20
+
+def test_complement():
+    assert f520.complement == SetClass(0, 1, 2, 5, 7, 8, 9)
+
+
+def test_complement_tonality():
+    assert f520.complement.tonality == 12
+
+
+def test_complement_cardinality():
+    assert f520.complement.cardinality == 7
+
+
+def test_complement_brightness():
+    assert f520.complement.brightness == 32
+
+
+def test_complement_pitch_classes():
+    assert len(f520.complement) == 7
+    assert len(f520.complement.pitch_classes) == 7
+
+
+def test_complement_adjacency_intervals():
+    assert len(f520.complement.adjacency_intervals) == 7
+
+
+def test_complement_versions():
+    assert len(f520.complement.versions) == 7
+
+
+def test_complement_brightest_form():
+    b = f520.complement.brightest_form
+    assert b in f520.complement.versions
+    assert b == SetClass(0, 3, 5, 6, 7, 10, 11)
+    assert b.leonard_notation == '[0₃3₂5₁6₁7₃10₁11₁]⁴²'
+
+
+def test_complement_darkest_form():
+    d = f520.complement.darkest_form
+    assert d in f520.complement.versions
+    assert d == SetClass(0, 1, 2, 5, 6, 7, 10)
+    assert d.leonard_notation == '[0₁1₁2₃5₁6₁7₃10₂]³¹'
+
+
+def test_complement_leonard_notation():
+    assert f520.complement.leonard_notation == '[0₁1₁2₃5₂7₁8₁9₃]³²'
+
+
+# ----------------------------------------------------------------------------
+# Forte 0-1, the empty set class
+empty = SetClass()
+
+
+def test_empty_tonality():
+    assert empty.tonality == 12
+
+
+def test_empty_cardinality():
+    assert empty.cardinality == 0
+
+
+def test_empty_brightness():
+    assert empty.brightness == 0
+
+
+def test_empty_pitch_classes():
+    assert len(empty) == 0
+    assert len(empty.pitch_classes) == 0
+
+
+def test_empty_ajdacency_intervals():
+    assert len(empty.adjacency_intervals) == 0
+
+
+def test_empty_versions():
+    assert len(empty.versions) == 1
+    assert empty.versions[0] == empty
+
+
+def test_empty_brightest_form():
+    assert empty.brightest_form == empty
+
+
+def test_empty_darkest_form():
+    assert empty.darkest_form == empty
+
+
+def test_empty_bright_dark_forms_equal():
+    assert empty.brightest_form == empty.darkest_form
+
+
+def test_empty_duodecimal_notation():
+    assert empty.duodecimal_notation == 'SetClass[]'
+
+
+def test_empty_dozenal_notation():
+    assert empty.dozenal_notation == 'SetClass[]'
+
+
+def test_empty_leonard_notation():
+    assert empty.leonard_notation == '[]⁰'
+
+
+# ----------------------------------------------------------------------------
+# Forte 12-1, the complement of empty (full set class)
+f121 = empty.complement
+
+
+def test_empty_complement():
+    assert f121 == SetClass(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
+
+
+def test_empty_complement_tonality():
+    assert f121.tonality == 12
+
+
+def test_empty_complement_cardinality():
+    assert f121.cardinality == 12
+
+
+def test_empty_complement_brightness():
+    assert f121.brightness == 66
+
+
+def test_empty_complement_pitch_classes():
+    assert len(f121) == 12
+    assert len(f121.pitch_classes) == 12
+
+
+def test_empty_complement_adjacency_intervals():
+    assert len(f121.adjacency_intervals) == 12
+
+
+def test_empty_complement_versions():
+    assert len(f121.versions) == 1
+    assert f121.versions[0] == f121
+
+
+def test_empty_complement_brightest_form():
+    assert f121.brightest_form == f121
+
+
+def test_empty_complement_darkest_form():
+    assert f121.darkest_form == f121
+
+
+def test_empty_complement_bright_dark_forms_equal():
+    assert f121.brightest_form == f121.darkest_form
+
+
+def test_empty_complement_duodecimal_notation():
+    assert f121.duodecimal_notation == 'SetClass[0,1,2,3,4,5,6,7,8,9,T,E]'
+
+
+def test_empty_complement_dozenal_notation():
+    assert f121.dozenal_notation == 'SetClass[0,1,2,3,4,5,6,7,8,9,↊,↋]'
+
+
+def test_empty_complement_leonard_notation():
+    assert f121.leonard_notation == '[0₁1₁2₁3₁4₁5₁6₁7₁8₁9₁10₁11₁]⁶⁶'
diff --git a/tox.ini b/tox.ini
index 3a0eecb..e286263 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,13 +6,18 @@ skip_missing_interpreters = True
 [testenv]
 skip_install = True
 setenv =
-    PYTHONPATH = {toxroot}
+    PYTHONPATH = {toxroot}/setclass
 commands =
     pytest --cov=. --junitxml=junit.xml --cov-report xml:coverage.xml --cov-report term {posargs}
 deps =
     -r requirements-dev.txt
 
 [coverage:run]
+omit =
+    ./**/tests/*.py
+    ./**/test_*.py
+    ./**/*_tests.py
+    ./**/tests.py
 
 [coverage:report]
 skip_empty = True