diff --git a/QuadrupleLib.Tests/Utilities/UInt256Tests.cs b/QuadrupleLib.Tests/Utilities/UInt256Tests.cs
new file mode 100644
index 0000000..17d46b3
--- /dev/null
+++ b/QuadrupleLib.Tests/Utilities/UInt256Tests.cs
@@ -0,0 +1,201 @@
+/*
+ *  Copyright 2026 Chosen Few Software
+ *  This file is part of QuadrupleLib.
+ *
+ *  QuadrupleLib is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Lesser General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  QuadrupleLib is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public License
+ *  along with QuadrupleLib.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+using QuadrupleLib.Utilities;
+using Xunit;
+
+namespace QuadrupleLib.Tests.Utilities
+{
+    public class UInt256Tests
+    {
+        [Fact]
+        public void IsAddWithoutCarryCorrect() 
+        {
+            UInt256 one = 1UL;
+            UInt256 two = 2UL;
+            Assert.Equal(3UL, one + two);
+        }
+
+        [Fact]
+        public void IsAddWithCarryCorrect()
+        {
+            UInt256 n = UInt256.One + UInt128.MaxValue;
+            Assert.Equal((UInt128.Zero, UInt128.One), (n._lo, n._hi));
+        }
+
+        [Fact]
+        public void IsAddWithOverflowZero()
+        {
+            UInt256 n = UInt256.MaxValue + UInt256.One;
+            Assert.Equal(UInt256.Zero, n);
+        }
+
+        [Fact]
+        public void IsSubtractWithoutBorrowCorrect()
+        {
+            UInt256 one = 1UL;
+            UInt256 three = 3UL;
+            Assert.Equal(2UL, three - one);
+        }
+
+        [Fact]
+        public void IsSubtractWithBorrowCorrect()
+        {
+            UInt256 n = UInt256.One + UInt128.MaxValue, m = n - UInt256.One;
+            Assert.Equal((UInt128.MaxValue, UInt128.Zero), (m._lo, m._hi));
+        }
+
+        [Fact]
+        public void IsSubtractWithUnderflowMaximum()
+        {
+            UInt256 n = UInt256.Zero - UInt256.One;
+            Assert.Equal(UInt256.MaxValue, n);
+        }
+
+        [Theory]
+        [InlineData(0UL, 0UL, 0)]
+        [InlineData(0UL, 0UL, 1)]
+        [InlineData(0UL, 0UL, 2)]
+        [InlineData(1UL, 1UL, 0)]
+        [InlineData(1UL, 2UL, 1)]
+        [InlineData(1UL, 4UL, 2)]
+        [InlineData(2UL, 2UL, 0)]
+        [InlineData(2UL, 4UL, 1)]
+        [InlineData(2UL, 8UL, 2)]
+        public void IsSmallShiftLeftCorrect(ulong n, ulong m, int amt)
+        {
+            UInt256 N = n; N <<= amt;
+            Assert.Equal(m, N);
+        }
+
+        [Theory]
+        [InlineData(0UL, 0UL, 0)]
+        [InlineData(0UL, 0UL, 1)]
+        [InlineData(0UL, 0UL, 2)]
+        [InlineData(1UL, 1UL, 0)]
+        [InlineData(1UL, 2UL, 1)]
+        [InlineData(1UL, 4UL, 2)]
+        [InlineData(2UL, 2UL, 0)]
+        [InlineData(2UL, 4UL, 1)]
+        [InlineData(2UL, 8UL, 2)]
+        public void IsLargeShiftLeftCorrect(ulong n, ulong m, int amt)
+        {
+            UInt256 N = n; N <<= 128 + amt;
+            UInt256 M = m; M <<= 128;
+            Assert.Equal(M, N);
+        }
+
+        [Theory]
+        [InlineData(0UL, 0)]
+        [InlineData(0UL, 1)]
+        [InlineData(0UL, 2)]
+        [InlineData(1UL, 0)]
+        [InlineData(1UL, 1)]
+        [InlineData(1UL, 2)]
+        [InlineData(2UL, 0)]
+        [InlineData(2UL, 1)]
+        [InlineData(2UL, 2)]
+        public void IsExtraLargeShiftLeftZero(ulong n, int amt)
+        {
+            UInt256 N = n; N <<= 256 + amt;
+            Assert.Equal(UInt256.Zero, N);
+        }
+
+        [Theory]
+        [InlineData(0UL, 0UL, 0)]
+        [InlineData(0UL, 0UL, 1)]
+        [InlineData(0UL, 0UL, 2)]
+        [InlineData(2UL, 2UL, 0)]
+        [InlineData(2UL, 1UL, 1)]
+        [InlineData(2UL, 0UL, 2)]
+        [InlineData(4UL, 4UL, 0)]
+        [InlineData(4UL, 2UL, 1)]
+        [InlineData(4UL, 1UL, 2)]
+        public void IsSmallShiftRightCorrect(ulong n, ulong m, int amt)
+        {
+            UInt256 N = n; N >>= amt;
+            Assert.Equal(m, N);
+        }
+
+        [Theory]
+        [InlineData(0UL, 0UL, 0)]
+        [InlineData(0UL, 0UL, 1)]
+        [InlineData(0UL, 0UL, 2)]
+        [InlineData(2UL, 2UL, 0)]
+        [InlineData(2UL, 1UL, 1)]
+        [InlineData(2UL, 0UL, 2)]
+        [InlineData(4UL, 4UL, 0)]
+        [InlineData(4UL, 2UL, 1)]
+        [InlineData(4UL, 1UL, 2)]
+        public void IsLargeShiftRightCorrect(ulong n, ulong m, int amt)
+        {
+            UInt256 N = (UInt256)n << 128; N >>= 128 + amt;
+            Assert.Equal(m, N);
+        }
+
+        [Theory]
+        [InlineData(0UL, 0)]
+        [InlineData(0UL, 1)]
+        [InlineData(0UL, 2)]
+        [InlineData(2UL, 0)]
+        [InlineData(2UL, 1)]
+        [InlineData(2UL, 2)]
+        [InlineData(4UL, 0)]
+        [InlineData(4UL, 1)]
+        [InlineData(4UL, 2)]
+        public void IsExtraLargeShiftRightZero(ulong n, int amt)
+        {
+            UInt256 N = (UInt256)n << 128; N >>= 256 + amt;
+            Assert.Equal(UInt256.Zero, N);
+        }
+
+        [Theory]
+        [InlineData(0UL, 0UL, 0)]
+        [InlineData(0UL, 1UL, -1)]
+        [InlineData(0UL, 2UL, -1)]
+        [InlineData(1UL, 0UL, 1)]
+        [InlineData(1UL, 1UL, 0)]
+        [InlineData(1UL, 2UL, -1)]
+        [InlineData(2UL, 0UL, 1)]
+        [InlineData(2UL, 1UL, 1)]
+        [InlineData(2UL, 2UL, 0)]
+        public void IsSmallCompareToCorrect(ulong n, ulong m, int s) 
+        {
+            UInt256 N = n, M = m;
+            int S = System.Math.Sign(N.CompareTo(M));
+            Assert.Equal(s, S);
+        }
+
+        [Theory]
+        [InlineData(0UL, 0UL, 0)]
+        [InlineData(0UL, 1UL, -1)]
+        [InlineData(0UL, 2UL, -1)]
+        [InlineData(1UL, 0UL, 1)]
+        [InlineData(1UL, 1UL, 0)]
+        [InlineData(1UL, 2UL, -1)]
+        [InlineData(2UL, 0UL, 1)]
+        [InlineData(2UL, 1UL, 1)]
+        [InlineData(2UL, 2UL, 0)]
+        public void IsLargeCompareToCorrect(ulong n, ulong m, int s)
+        {
+            UInt256 N = (UInt256)n << 128, M = (UInt256)m << 128;
+            int S = System.Math.Sign(N.CompareTo(M));
+            Assert.Equal(s, S);
+        }
+    }
+}
diff --git a/QuadrupleLib/AssemblyInfo.cs b/QuadrupleLib/AssemblyInfo.cs
new file mode 100644
index 0000000..1c0e937
--- /dev/null
+++ b/QuadrupleLib/AssemblyInfo.cs
@@ -0,0 +1,3 @@
+using System.Runtime.CompilerServices;
+
+[assembly: InternalsVisibleTo("QuadrupleLib.Tests")]
diff --git a/QuadrupleLib/UInt256.cs b/QuadrupleLib/Utilities/UInt256.cs
similarity index 94%
rename from QuadrupleLib/UInt256.cs
rename to QuadrupleLib/Utilities/UInt256.cs
index 8c8a31e..06e74dc 100644
--- a/QuadrupleLib/UInt256.cs
+++ b/QuadrupleLib/Utilities/UInt256.cs
@@ -16,8 +16,12 @@
  *  along with QuadrupleLib.  If not, see <https://www.gnu.org/licenses/>.
  */
 
-namespace QuadrupleLib
+using System.Numerics;
+using System.Runtime.InteropServices;
+
+namespace QuadrupleLib.Utilities
 {
+    [StructLayout(LayoutKind.Sequential)]
     internal struct UInt256 : IEquatable<UInt256>, IComparable<UInt256>, IComparable
     {
         private static readonly UInt256 _zero = new();
@@ -34,11 +38,11 @@ internal struct UInt256 : IEquatable<UInt256>, IComparable<UInt256>, IComparable
 
 
 #if BIGENDIAN
-        private readonly UInt128 _hi;
-        private readonly UInt128 _lo;
+        public readonly UInt128 _hi;
+        public readonly UInt128 _lo;
 #else
-        private readonly UInt128 _lo;
-        private readonly UInt128 _hi;
+        public readonly UInt128 _lo;
+        public readonly UInt128 _hi;
 #endif
         private UInt256(UInt128 lo, UInt128 hi) 
         {
@@ -263,5 +267,12 @@ public override int GetHashCode()
         {
             return HashCode.Combine(_lo, _hi);
         }
+
+        public override string ToString()
+        {
+            BigInteger n = _hi; 
+            n = (n << 128) | _lo;
+            return n.ToString();
+        }
     }
 }
\ No newline at end of file