diff --git a/README.md b/README.md
index ea03e19..1594fd5 100644
--- a/README.md
+++ b/README.md
@@ -3,3 +3,13 @@
 Python port of [KeySet in TypeScript](https://github.com/eturino/ts-key-set) and [KeySet in Ruby](https://github.com/eturino/ruby_key_set)
 
 TBD
+
+## `KeySetType` enum
+
+values
+
+- `ALL` represents the entirety of possible keys (`𝕌`)
+- `NONE` represents an empty set (`∅`)
+- `SOME` represents a concrete set (`A ⊂ 𝕌`)
+- `ALL_EXCEPT_SOME` represents the complementary of a set, all the elements except the given ones (`A' = {x ∈ 𝕌 | x ∉ A}`) _(see [Complement in Wikipedia](https://en.wikipedia.org/wiki/Complement_set_theory))*
+
diff --git a/key_set/enum.py b/key_set/enum.py
new file mode 100644
index 0000000..c8e9b39
--- /dev/null
+++ b/key_set/enum.py
@@ -0,0 +1,8 @@
+from enum import Enum
+
+
+class KeySetType(Enum):
+    ALL = 'ALL'
+    ALL_EXCEPT_SOME = 'ALL_EXCEPT_SOME'
+    NONE = 'NONE'
+    SOME = 'SOME'
diff --git a/tests/test_enum.py b/tests/test_enum.py
new file mode 100644
index 0000000..b771ada
--- /dev/null
+++ b/tests/test_enum.py
@@ -0,0 +1,28 @@
+# -*- coding: utf-8 -*-
+"""Enum test
+Check the enum
+"""
+
+import __future__  # noqa: F401
+
+import json  # noqa: F401
+from os import path  # noqa: F401
+from re import IGNORECASE, sub  # noqa: F401
+
+import key_set  # noqa: F401
+from key_set.enum import KeySetType
+
+
+class TestEnum:  # noqa: D101
+
+    def test_all(self) -> None:
+        assert KeySetType.ALL.value == 'ALL'
+
+    def test_some(self) -> None:
+        assert KeySetType.SOME.value == 'SOME'
+
+    def test_none(self) -> None:
+        assert KeySetType.NONE.value == 'NONE'
+
+    def test_all_except_some(self) -> None:
+        assert KeySetType.ALL_EXCEPT_SOME.value == 'ALL_EXCEPT_SOME'