diff --git a/Semantics.Test/Quantities/SemanticOverloadTests.cs b/Semantics.Test/Quantities/SemanticOverloadTests.cs
new file mode 100644
index 0000000..0d256af
--- /dev/null
+++ b/Semantics.Test/Quantities/SemanticOverloadTests.cs
@@ -0,0 +1,151 @@
+// Copyright (c) ktsu.dev
+// All rights reserved.
+// Licensed under the MIT license.
+
+namespace ktsu.Semantics.Test.Quantities;
+
+using ktsu.Semantics.Quantities;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Covers semantic overload conversions and metadata-driven relationships.
+/// Issue #55.
+/// </summary>
+[TestClass]
+public sealed class SemanticOverloadTests
+{
+	private const double Tolerance = 1e-10;
+
+	// ----------------------------------------- Implicit widening to base
+
+	[TestMethod]
+	public void Weight_Widens_Implicitly_To_ForceMagnitude()
+	{
+		Weight<double> w = Weight<double>.FromNewton(686.0);
+		ForceMagnitude<double> baseValue = w; // implicit conversion
+		Assert.AreEqual(686.0, baseValue.Value, Tolerance);
+	}
+
+	[TestMethod]
+	public void Distance_Widens_Implicitly_To_Length()
+	{
+		Distance<double> d = Distance<double>.FromMeter(42.0);
+		Length<double> len = d;
+		Assert.AreEqual(42.0, len.Value, Tolerance);
+	}
+
+	[TestMethod]
+	public void Diameter_Widens_Implicitly_To_Length()
+	{
+		Diameter<double> diam = Diameter<double>.FromMeter(10.0);
+		Length<double> len = diam;
+		Assert.AreEqual(10.0, len.Value, Tolerance);
+	}
+
+	// ---------------------------------------- Explicit narrowing from base
+
+	[TestMethod]
+	public void ForceMagnitude_Narrows_Explicitly_To_Weight()
+	{
+		ForceMagnitude<double> fm = ForceMagnitude<double>.FromNewton(686.0);
+		Weight<double> w = (Weight<double>)fm;
+		Assert.AreEqual(686.0, w.Value, Tolerance);
+	}
+
+	[TestMethod]
+	public void Length_Narrows_Explicitly_To_Distance()
+	{
+		Length<double> len = Length<double>.FromMeter(42.0);
+		Distance<double> d = (Distance<double>)len;
+		Assert.AreEqual(42.0, d.Value, Tolerance);
+	}
+
+	// --------------------------------------------- From(base) factory
+
+	[TestMethod]
+	public void Weight_From_ForceMagnitude_Constructs()
+	{
+		ForceMagnitude<double> fm = ForceMagnitude<double>.FromNewton(100.0);
+		Weight<double> w = Weight<double>.From(fm);
+		Assert.AreEqual(100.0, w.Value, Tolerance);
+	}
+
+	[TestMethod]
+	public void Distance_From_Length_Constructs()
+	{
+		Length<double> len = Length<double>.FromMeter(7.0);
+		Distance<double> d = Distance<double>.From(len);
+		Assert.AreEqual(7.0, d.Value, Tolerance);
+	}
+
+	// -------------------- Round-trip widen/narrow preserves value
+
+	[TestMethod]
+	public void Weight_RoundTrip_Through_ForceMagnitude_Preserves_Value()
+	{
+		Weight<double> original = Weight<double>.FromNewton(123.456);
+		ForceMagnitude<double> widened = original;
+		Weight<double> narrowed = (Weight<double>)widened;
+		Assert.AreEqual(original.Value, narrowed.Value, Tolerance);
+	}
+
+	// ------------------ Metadata-defined relationship: Diameter <-> Radius
+
+	[TestMethod]
+	public void Diameter_ToRadius_Halves_Value()
+	{
+		Diameter<double> d = Diameter<double>.FromMeter(10.0);
+		Radius<double> r = d.ToRadius();
+		Assert.AreEqual(5.0, r.Value, Tolerance);
+	}
+
+	[TestMethod]
+	public void Diameter_FromRadius_Doubles_Value()
+	{
+		Radius<double> r = Radius<double>.FromMeter(5.0);
+		Diameter<double> d = Diameter<double>.FromRadius(r);
+		Assert.AreEqual(10.0, d.Value, Tolerance);
+	}
+
+	[TestMethod]
+	public void Diameter_RoundTrip_Through_Radius_Preserves_Value()
+	{
+		Diameter<double> d = Diameter<double>.FromMeter(20.0);
+		Radius<double> r = d.ToRadius();
+		Diameter<double> back = Diameter<double>.FromRadius(r);
+		Assert.AreEqual(d.Value, back.Value, Tolerance);
+	}
+
+	// ----------------- V0 overload subtraction
+	// Locked in #52: V0 - V0 returns the same V0 of T.Abs(a - b).
+	// Generator currently emits a Force1D-returning subtraction for Weight - Weight,
+	// which violates that rule. The current behaviour is documented here so the fix
+	// in #52 can replace this test with the correct shape.
+
+	[TestMethod]
+	public void Weight_Minus_Weight_Currently_Returns_Force1D_PendingFix52()
+	{
+		Weight<double> a = Weight<double>.FromNewton(100.0);
+		Weight<double> b = Weight<double>.FromNewton(150.0);
+		Force1D<double> diff = a - b; // current generator behaviour; #52 plans Weight of |a - b|.
+		Assert.AreEqual(-50.0, diff.Value, Tolerance);
+	}
+
+	// ----------------- Storage-type genericity sanity
+
+	[TestMethod]
+	public void Diameter_ToRadius_Works_With_Float_Storage()
+	{
+		Diameter<float> d = Diameter<float>.FromMeter(10.0f);
+		Radius<float> r = d.ToRadius();
+		Assert.AreEqual(5.0f, r.Value, 1e-6f);
+	}
+
+	[TestMethod]
+	public void Diameter_ToRadius_Works_With_Decimal_Storage()
+	{
+		Diameter<decimal> d = Diameter<decimal>.FromMeter(10m);
+		Radius<decimal> r = d.ToRadius();
+		Assert.AreEqual(5m, r.Value);
+	}
+}
diff --git a/Semantics.Test/Quantities/VectorQuantityTests.cs b/Semantics.Test/Quantities/VectorQuantityTests.cs
new file mode 100644
index 0000000..492952d
--- /dev/null
+++ b/Semantics.Test/Quantities/VectorQuantityTests.cs
@@ -0,0 +1,212 @@
+// Copyright (c) ktsu.dev
+// All rights reserved.
+// Licensed under the MIT license.
+
+namespace ktsu.Semantics.Test.Quantities;
+
+using ktsu.Semantics.Quantities;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Covers <see cref="IVector0{TSelf, T}"/>..<see cref="IVector4{TSelf, T}"/> contracts:
+/// magnitude extraction, typed dot/cross products, vector arithmetic, and V0 invariants.
+/// Issue #54.
+/// </summary>
+[TestClass]
+public sealed class VectorQuantityTests
+{
+	private const double Tolerance = 1e-10;
+
+	// -------------------------------------------------------------- Magnitude
+
+	[TestMethod]
+	public void Velocity3D_Magnitude_Of_3_4_0_Is_Speed_5()
+	{
+		Velocity3D<double> v = new() { X = 3.0, Y = 4.0, Z = 0.0 };
+		Speed<double> s = v.Magnitude();
+		Assert.AreEqual(5.0, s.Value, Tolerance);
+	}
+
+	[TestMethod]
+	public void Force3D_Magnitude_Is_Always_NonNegative_Even_With_Negative_Components()
+	{
+		Force3D<double> f = new() { X = -3.0, Y = -4.0, Z = 0.0 };
+		ForceMagnitude<double> m = f.Magnitude();
+		Assert.AreEqual(5.0, m.Value, Tolerance);
+	}
+
+	[TestMethod]
+	public void Velocity3D_Magnitude_Returns_Speed_Type_Statically()
+	{
+		Velocity3D<double> v = new() { X = 1.0, Y = 0.0, Z = 0.0 };
+		Speed<double> s = v.Magnitude();
+		Assert.IsInstanceOfType<Speed<double>>(s);
+	}
+
+	[TestMethod]
+	public void Velocity3D_Magnitude_Of_Zero_Vector_Is_Zero()
+	{
+		Velocity3D<double> zero = Velocity3D<double>.Zero;
+		Speed<double> s = zero.Magnitude();
+		Assert.AreEqual(0.0, s.Value, Tolerance);
+	}
+
+	// ------------------------------------------------------ Typed dot product
+
+	[TestMethod]
+	public void Force3D_Dot_Displacement3D_Returns_Energy_Aligned()
+	{
+		Force3D<double> f = new() { X = 10.0, Y = 0.0, Z = 0.0 };
+		Displacement3D<double> r = new() { X = 2.0, Y = 0.0, Z = 0.0 };
+		Energy<double> work = f.Dot(r);
+		Assert.AreEqual(20.0, work.Value, Tolerance);
+	}
+
+	[TestMethod]
+	public void Force3D_Dot_Displacement3D_Is_Zero_For_Perpendicular()
+	{
+		Force3D<double> f = new() { X = 10.0, Y = 0.0, Z = 0.0 };
+		Displacement3D<double> r = new() { X = 0.0, Y = 5.0, Z = 0.0 };
+		Energy<double> work = f.Dot(r);
+		Assert.AreEqual(0.0, work.Value, Tolerance);
+	}
+
+	// ---------------------------------------------------- Typed cross product
+
+	[TestMethod]
+	public void Force3D_Cross_Displacement3D_Returns_Torque3D()
+	{
+		Force3D<double> f = new() { X = 0.0, Y = 10.0, Z = 0.0 };
+		Displacement3D<double> r = new() { X = 0.5, Y = 0.0, Z = 0.0 };
+		Torque3D<double> t = f.Cross(r);
+		// (Y*rZ - Z*rY, Z*rX - X*rZ, X*rY - Y*rX) = (0, 0, -5)
+		Assert.AreEqual(0.0, t.X, Tolerance);
+		Assert.AreEqual(0.0, t.Y, Tolerance);
+		Assert.AreEqual(-5.0, t.Z, Tolerance);
+	}
+
+	[TestMethod]
+	public void Force3D_Cross_Self_Is_Zero_Vector()
+	{
+		Force3D<double> f = new() { X = 1.0, Y = 2.0, Z = 3.0 };
+		// Same-dimension structural cross returns Force3D (the dimension itself, not a typed dimensional product).
+		Force3D<double> c = f.Cross(f);
+		Assert.AreEqual(0.0, c.X, Tolerance);
+		Assert.AreEqual(0.0, c.Y, Tolerance);
+		Assert.AreEqual(0.0, c.Z, Tolerance);
+	}
+
+	// --------------------------------------------- Same-dimension dot product
+
+	[TestMethod]
+	public void Velocity3D_Dot_Velocity3D_Returns_Raw_Storage_Scalar()
+	{
+		// Same-dimension Dot is structural and returns the raw storage type; it isn't
+		// a typed dimensional product (no "Speed²" exists in the type system).
+		Velocity3D<double> a = new() { X = 1.0, Y = 2.0, Z = 3.0 };
+		Velocity3D<double> b = new() { X = 4.0, Y = 5.0, Z = 6.0 };
+		double dot = a.Dot(b);
+		Assert.AreEqual(32.0, dot, Tolerance);
+	}
+
+	// ------------------------------------------------ Vector form arithmetic
+
+	[TestMethod]
+	public void Force3D_Plus_Force3D_Stays_Force3D_Componentwise()
+	{
+		Force3D<double> a = new() { X = 1.0, Y = 2.0, Z = 3.0 };
+		Force3D<double> b = new() { X = 4.0, Y = 5.0, Z = 6.0 };
+		Force3D<double> sum = a + b;
+		Assert.AreEqual(5.0, sum.X, Tolerance);
+		Assert.AreEqual(7.0, sum.Y, Tolerance);
+		Assert.AreEqual(9.0, sum.Z, Tolerance);
+	}
+
+	[TestMethod]
+	public void Force3D_Minus_Force3D_Componentwise()
+	{
+		Force3D<double> a = new() { X = 5.0, Y = 7.0, Z = 9.0 };
+		Force3D<double> b = new() { X = 1.0, Y = 2.0, Z = 3.0 };
+		Force3D<double> diff = a - b;
+		Assert.AreEqual(4.0, diff.X, Tolerance);
+		Assert.AreEqual(5.0, diff.Y, Tolerance);
+		Assert.AreEqual(6.0, diff.Z, Tolerance);
+	}
+
+	[TestMethod]
+	public void Force3D_Negation_Inverts_Each_Component()
+	{
+		Force3D<double> f = new() { X = 1.0, Y = -2.0, Z = 3.0 };
+		Force3D<double> n = -f;
+		Assert.AreEqual(-1.0, n.X, Tolerance);
+		Assert.AreEqual(2.0, n.Y, Tolerance);
+		Assert.AreEqual(-3.0, n.Z, Tolerance);
+	}
+
+	// ------------------------------------------------------------- V0 + V0
+
+	[TestMethod]
+	public void Mass_Plus_Mass_Returns_Mass()
+	{
+		Mass<double> a = Mass<double>.FromKilogram(3.0);
+		Mass<double> b = Mass<double>.FromKilogram(5.0);
+		Mass<double> sum = a + b;
+		Assert.AreEqual(8.0, sum.Value, Tolerance);
+		Assert.IsInstanceOfType<Mass<double>>(sum);
+	}
+
+	[TestMethod]
+	public void Speed_Plus_Speed_Returns_Speed()
+	{
+		Speed<double> a = Speed<double>.FromMetersPerSecond(3.0);
+		Speed<double> b = Speed<double>.FromMetersPerSecond(5.0);
+		Speed<double> sum = a + b;
+		Assert.AreEqual(8.0, sum.Value, Tolerance);
+	}
+
+	// ------------------------------------------------------------- V0 - V0
+	// Locked design decision in #52: V0 - V0 should return the same V0 of T.Abs(a - b).
+	// Generator currently emits unsigned subtraction via the SemanticQuantity base, which
+	// can produce a negative magnitude. Tracked as a follow-up.
+
+	[TestMethod]
+	[Ignore("Locked in #52: V0 - V0 should return the same V0 of T.Abs(a - b). Generator currently emits unsigned subtraction.")]
+	public void Mass_Minus_Mass_Returns_Absolute_Difference_Pending52()
+	{
+		Mass<double> a = Mass<double>.FromKilogram(3.0);
+		Mass<double> b = Mass<double>.FromKilogram(5.0);
+		Mass<double> diff = a - b;
+		Assert.AreEqual(2.0, diff.Value, Tolerance);
+	}
+
+	// ---------------------------------------------------- V0 non-negativity
+	// Tracked in #50: factories on Vector0 quantities should reject negative inputs
+	// with ArgumentException. The current generator does not emit guards.
+
+	[TestMethod]
+	[Ignore("Tracked in #50: V0 factories should reject negative inputs.")]
+	public void Speed_From_Negative_Throws_Pending50()
+	{
+		_ = Assert.ThrowsExactly<System.ArgumentException>(
+			() => Speed<double>.FromMetersPerSecond(-1.0));
+	}
+
+	[TestMethod]
+	[Ignore("Tracked in #50: V0 factories should reject negative inputs.")]
+	public void Mass_From_Negative_Throws_Pending50()
+	{
+		_ = Assert.ThrowsExactly<System.ArgumentException>(
+			() => Mass<double>.FromKilogram(-1.0));
+	}
+
+	// -------------------------------------------------- Magnitude on V1
+	// Velocity1D.Magnitude() should return Speed of T.Abs(value).
+
+	[TestMethod]
+	public void Velocity1D_Magnitude_Of_Negative_Is_Positive_Speed()
+	{
+		Velocity1D<double> v = Velocity1D<double>.FromMetersPerSecond(-3.5);
+		Speed<double> s = v.Magnitude();
+		Assert.AreEqual(3.5, s.Value, Tolerance);
+	}
+}