diff --git a/X10D.Tests/src/Math/ByteTests.cs b/X10D.Tests/src/Math/ByteTests.cs
index 6608222..ac1b99e 100644
--- a/X10D.Tests/src/Math/ByteTests.cs
+++ b/X10D.Tests/src/Math/ByteTests.cs
@@ -51,13 +51,29 @@ internal partial class ByteTests
     }
 
     [Test]
-    public void DigitalRootShouldBeCorrect()
+    public void DigitalRoot_ShouldReturn4_Given238()
     {
         const byte value = 238;
         Assert.That(value.DigitalRoot(), Is.EqualTo(4));
         Assert.That((-value).DigitalRoot(), Is.EqualTo(4));
     }
 
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given9()
+    {
+        const byte value = 9;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given18()
+    {
+        const byte value = 18;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
     [Test]
     public void FactorialShouldBeCorrect()
     {
diff --git a/X10D.Tests/src/Math/Int16Tests.cs b/X10D.Tests/src/Math/Int16Tests.cs
index 7ba50a2..ac68889 100644
--- a/X10D.Tests/src/Math/Int16Tests.cs
+++ b/X10D.Tests/src/Math/Int16Tests.cs
@@ -62,13 +62,29 @@ internal partial class Int16Tests
     }
 
     [Test]
-    public void DigitalRootShouldBeCorrect()
+    public void DigitalRoot_ShouldReturn4_Given238()
     {
         const short value = 238;
         Assert.That(value.DigitalRoot(), Is.EqualTo(4));
         Assert.That((-value).DigitalRoot(), Is.EqualTo(4));
     }
 
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given9()
+    {
+        const short value = 9;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given18()
+    {
+        const short value = 18;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
     [Test]
     public void FactorialShouldBeCorrect()
     {
diff --git a/X10D.Tests/src/Math/Int32Tests.cs b/X10D.Tests/src/Math/Int32Tests.cs
index 664e16f..68e05cd 100644
--- a/X10D.Tests/src/Math/Int32Tests.cs
+++ b/X10D.Tests/src/Math/Int32Tests.cs
@@ -62,13 +62,29 @@ internal partial class Int32Tests
     }
 
     [Test]
-    public void DigitalRootShouldBeCorrect()
+    public void DigitalRoot_ShouldReturn4_Given238()
     {
         const int value = 238;
         Assert.That(value.DigitalRoot(), Is.EqualTo(4));
         Assert.That((-value).DigitalRoot(), Is.EqualTo(4));
     }
 
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given9()
+    {
+        const int value = 9;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given18()
+    {
+        const int value = 18;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
     [Test]
     public void FactorialShouldBeCorrect()
     {
diff --git a/X10D.Tests/src/Math/Int64Tests.cs b/X10D.Tests/src/Math/Int64Tests.cs
index 8dd40a9..e1f37fd 100644
--- a/X10D.Tests/src/Math/Int64Tests.cs
+++ b/X10D.Tests/src/Math/Int64Tests.cs
@@ -62,13 +62,29 @@ internal partial class Int64Tests
     }
 
     [Test]
-    public void DigitalRootShouldBeCorrect()
+    public void DigitalRoot_ShouldReturn4_Given238()
     {
         const long value = 238;
         Assert.That(value.DigitalRoot(), Is.EqualTo(4));
         Assert.That((-value).DigitalRoot(), Is.EqualTo(4));
     }
 
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given9()
+    {
+        const long value = 9;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given18()
+    {
+        const long value = 18;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
     [Test]
     public void FactorialShouldBeCorrect()
     {
diff --git a/X10D.Tests/src/Math/SByteTests.cs b/X10D.Tests/src/Math/SByteTests.cs
index 3727af4..fd9a4fa 100644
--- a/X10D.Tests/src/Math/SByteTests.cs
+++ b/X10D.Tests/src/Math/SByteTests.cs
@@ -62,13 +62,29 @@ internal partial class SByteTests
     }
 
     [Test]
-    public void DigitalRootShouldBeCorrect()
+    public void DigitalRoot_ShouldReturn4_Given127()
     {
         const sbyte value = 127; // sbyte.MaxValue. can't use 238 like the other tests
         Assert.That(value.DigitalRoot(), Is.EqualTo(1));
         Assert.That((-value).DigitalRoot(), Is.EqualTo(1));
     }
 
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given9()
+    {
+        const sbyte value = 9;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given18()
+    {
+        const sbyte value = 18;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
     [Test]
     public void FactorialShouldBeCorrect()
     {
diff --git a/X10D.Tests/src/Math/UInt16Tests.cs b/X10D.Tests/src/Math/UInt16Tests.cs
index d51f983..3a7bb0d 100644
--- a/X10D.Tests/src/Math/UInt16Tests.cs
+++ b/X10D.Tests/src/Math/UInt16Tests.cs
@@ -51,13 +51,29 @@ internal partial class UInt16Tests
     }
 
     [Test]
-    public void DigitalRootShouldBeCorrect()
+    public void DigitalRoot_ShouldReturn4_Given238()
     {
         const ushort value = 238;
         Assert.That(value.DigitalRoot(), Is.EqualTo(4));
         Assert.That((-value).DigitalRoot(), Is.EqualTo(4));
     }
 
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given9()
+    {
+        const ushort value = 9;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given18()
+    {
+        const ushort value = 18;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
     [Test]
     public void FactorialShouldBeCorrect()
     {
diff --git a/X10D.Tests/src/Math/UInt32Tests.cs b/X10D.Tests/src/Math/UInt32Tests.cs
index f2db8e8..e5c9e91 100644
--- a/X10D.Tests/src/Math/UInt32Tests.cs
+++ b/X10D.Tests/src/Math/UInt32Tests.cs
@@ -50,11 +50,27 @@ internal partial class UInt32Tests
     }
 
     [Test]
-    public void DigitalRootShouldBeCorrect()
+    public void DigitalRoot_ShouldReturn4_Given238()
     {
         const uint value = 238;
-        Assert.That(value.DigitalRoot(), Is.EqualTo(4U));
-        Assert.That((-value).DigitalRoot(), Is.EqualTo(4U));
+        Assert.That(value.DigitalRoot(), Is.EqualTo(4));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(4));
+    }
+
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given9()
+    {
+        const uint value = 9;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
+    }
+
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given18()
+    {
+        const uint value = 18;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-value).DigitalRoot(), Is.EqualTo(9));
     }
 
     [Test]
diff --git a/X10D.Tests/src/Math/UInt64Tests.cs b/X10D.Tests/src/Math/UInt64Tests.cs
index 7391cc5..8c7a215 100644
--- a/X10D.Tests/src/Math/UInt64Tests.cs
+++ b/X10D.Tests/src/Math/UInt64Tests.cs
@@ -51,15 +51,31 @@ internal partial class UInt64Tests
     }
 
     [Test]
-    public void DigitalRootShouldBeCorrect()
+    public void DigitalRoot_ShouldReturn4_Given238()
     {
         const ulong value = 238;
-        Assert.That(value.DigitalRoot(), Is.EqualTo(4U));
+        Assert.That(value.DigitalRoot(), Is.EqualTo(4));
 
         // -ulong operator not defined because it might exceed long.MinValue,
         // so instead, cast to long and then negate.
         // HAX.
-        Assert.That((-(long)value).DigitalRoot(), Is.EqualTo(4U));
+        Assert.That((-(long)value).DigitalRoot(), Is.EqualTo(4));
+    }
+
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given9()
+    {
+        const ulong value = 9;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-(long)value).DigitalRoot(), Is.EqualTo(9));
+    }
+
+    [Test]
+    public void DigitalRoot_ShouldReturn9_Given18()
+    {
+        const ulong value = 18;
+        Assert.That(value.DigitalRoot(), Is.EqualTo(9));
+        Assert.That((-(long)value).DigitalRoot(), Is.EqualTo(9));
     }
 
     [Test]