diff --git a/.gitignore b/.gitignore
index c16d3d4..4faa3d5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,14 +2,25 @@
 *~
 .*.sw[op]
 *.egg-info
+*.dist-info
+.eggs
+.coverage
+.coverage.*
+coverage.xml
 dist
 build
+logs
 _build
-local
 distribute-*
-.coverage
+docs/env
+local
+*venv*
+.idea
+Pipfile*
 .tox
+.DS_Store
+pip-selfcheck.json
+pyvenv.cfg
 .vscode
-venv*
-.idea
+.envrc
 pip-wheel-metadata
diff --git a/changes/143.feature.rst b/changes/143.feature.rst
new file mode 100644
index 0000000..234ac89
--- /dev/null
+++ b/changes/143.feature.rst
@@ -0,0 +1 @@
+BaseStyle now supports |, |=, and 'in' operators.
diff --git a/src/travertino/declaration.py b/src/travertino/declaration.py
index cec23be..6b02965 100644
--- a/src/travertino/declaration.py
+++ b/src/travertino/declaration.py
@@ -1,4 +1,5 @@
 from collections import defaultdict
+from typing import Mapping
 from warnings import filterwarnings, warn
 
 from .colors import color
@@ -142,6 +143,9 @@ def __delete__(self, obj):
         else:
             obj.apply(self.name, self.initial)
 
+    def is_set_on(self, obj):
+        return hasattr(obj, f"_{self.name}")
+
 
 class directional_property:
     DIRECTIONS = [TOP, RIGHT, BOTTOM, LEFT]
@@ -199,6 +203,11 @@ def __delete__(self, obj):
         for direction in self.DIRECTIONS:
             del obj[self.format(direction)]
 
+    def is_set_on(self, obj):
+        return any(
+            hasattr(obj, self.format(direction)) for direction in self.DIRECTIONS
+        )
+
 
 class BaseStyle:
     """A base class for style declarations.
@@ -282,6 +291,13 @@ def __delitem__(self, name):
         else:
             raise KeyError(name)
 
+    def keys(self):
+        return {
+            name
+            for name in self._PROPERTIES[self.__class__]
+            if hasattr(self, f"_{name}")
+        }
+
     def items(self):
         return [
             (name, value)
@@ -289,12 +305,43 @@ def items(self):
             if (value := getattr(self, f"_{name}", None)) is not None
         ]
 
-    def keys(self):
-        return {
+    def __len__(self):
+        return sum(
+            1 for name in self._PROPERTIES[self.__class__] if hasattr(self, f"_{name}")
+        )
+
+    def __contains__(self, name):
+        return name in self._ALL_PROPERTIES[self.__class__] and (
+            getattr(self.__class__, name).is_set_on(self)
+        )
+
+    def __iter__(self):
+        yield from (
             name
             for name in self._PROPERTIES[self.__class__]
             if hasattr(self, f"_{name}")
-        }
+        )
+
+    def __or__(self, other):
+        if isinstance(other, BaseStyle):
+            if self.__class__ is not other.__class__:
+                return NotImplemented
+        elif not isinstance(other, Mapping):
+            return NotImplemented
+
+        result = self.copy()
+        result.update(**other)
+        return result
+
+    def __ior__(self, other):
+        if isinstance(other, BaseStyle):
+            if self.__class__ is not other.__class__:
+                return NotImplemented
+        elif not isinstance(other, Mapping):
+            return NotImplemented
+
+        self.update(**other)
+        return self
 
     ######################################################################
     # Get the rendered form of the style declaration
diff --git a/tests/test_declaration.py b/tests/test_declaration.py
index 7a1da8e..0d76ec6 100644
--- a/tests/test_declaration.py
+++ b/tests/test_declaration.py
@@ -73,6 +73,18 @@ class DeprecatedStyle(BaseStyle):
     DeprecatedStyle.directional_property("thing%s")
 
 
+class StyleSubclass(Style):
+    pass
+
+
+class DeprecatedStyleSubclass(DeprecatedStyle):
+    pass
+
+
+class Sibling(BaseStyle):
+    pass
+
+
 def test_invalid_style():
     with pytest.raises(ValueError):
         # Define an invalid initial value on a validated property
@@ -504,7 +516,7 @@ def test_dict(StyleClass):
         thing=(30, 40, 50, 60),
     )
 
-    assert style.keys() == {
+    expected_keys = {
         "explicit_const",
         "explicit_value",
         "thing_bottom",
@@ -512,6 +524,9 @@ def test_dict(StyleClass):
         "thing_right",
         "thing_top",
     }
+
+    assert style.keys() == expected_keys
+
     assert sorted(style.items()) == sorted(
         [
             ("explicit_const", "value2"),
@@ -523,6 +538,20 @@ def test_dict(StyleClass):
         ]
     )
 
+    # Properties that are set are in the keys.
+    for name in expected_keys:
+        assert name in style
+
+    # Directional properties with one or more of the aliased properties set also count.
+    assert "thing" in style
+
+    # Valid properties that haven't been set are not in the keys.
+    assert "implicit" not in style
+    assert "explicit_none" not in style
+
+    # Neither are invalid properties.
+    assert "invalid_property" not in style
+
     # A property can be set, retrieved and cleared using the attribute name
     style["thing-bottom"] = 10
     assert style["thing-bottom"] == 10
@@ -550,6 +579,89 @@ def test_dict(StyleClass):
         del style["no-such-property"]
 
 
+@pytest.mark.parametrize("StyleClass", [Style, DeprecatedStyle])
+@pytest.mark.parametrize("instantiate", [True, False])
+def test_union_operators(StyleClass, instantiate):
+    """Styles support | and |= with dicts and with their own class."""
+    left = StyleClass(explicit_value=VALUE1, implicit=VALUE2)
+
+    style_dict = {"thing_top": 5, "implicit": VALUE3}
+    right = StyleClass(**style_dict) if instantiate else style_dict
+
+    # Standard operator
+    result = left | right
+
+    # Original objects unchanged
+    assert left["explicit_value"] == VALUE1
+    assert left["implicit"] == VALUE2
+
+    assert right["thing_top"] == 5
+    assert right["implicit"] == VALUE3
+
+    # Unshared properties assigned
+    assert result["explicit_const"] == VALUE1
+    assert result["thing_top"] == 5
+
+    # Common property overridden by second operand
+    assert result["implicit"] == VALUE3
+
+    # In-place version
+    left |= right
+
+    # Common property updated on lefthand
+    assert left["explicit_value"] == VALUE1
+    assert left["implicit"] == VALUE3
+
+    # Righthand unchanged
+    assert right["thing_top"] == 5
+    assert right["implicit"] == VALUE3
+
+
+@pytest.mark.parametrize(
+    "StyleClass, OtherClass",
+    [
+        (Style, StyleSubclass),
+        (Style, Sibling),
+        (Style, int),
+        (Style, list),
+        (DeprecatedStyle, DeprecatedStyleSubclass),
+        (DeprecatedStyle, Sibling),
+        (DeprecatedStyle, int),
+        (DeprecatedStyle, list),
+    ],
+)
+def test_union_operators_invalid_type(StyleClass, OtherClass):
+    """Styles do not support | or |= with other style classes or with non-mappings."""
+
+    left = StyleClass()
+    right = OtherClass()
+
+    with pytest.raises(TypeError, match=r"unsupported operand type"):
+        left | right
+
+    with pytest.raises(TypeError, match=r"unsupported operand type"):
+        left |= right
+
+
+@pytest.mark.parametrize("StyleClass", [Style, DeprecatedStyle])
+@pytest.mark.parametrize(
+    "right, error",
+    [
+        ({"implicit": "bogus_value"}, ValueError),
+        ({"bogus_key": 3.12}, NameError),
+    ],
+)
+def test_union_operators_invalid_key_value(StyleClass, right, error):
+    """Operators will accept any mapping, but invalid keys/values are still an error."""
+    left = StyleClass()
+
+    with pytest.raises(error):
+        left | right
+
+    with pytest.raises(error):
+        left |= right
+
+
 def test_deprecated_class_methods():
     class OldStyle(BaseStyle):
         pass