diff --git a/src/Core/CayleyDickson.fs b/src/Core/CayleyDickson.fs
new file mode 100644
index 0000000000..681732092f
--- /dev/null
+++ b/src/Core/CayleyDickson.fs
@@ -0,0 +1,133 @@
+namespace Zeta.Core
+
+/// Cayley–Dickson doubling — the structural primitive underlying the
+/// "imaginary stack" design captured in the B-0623 trajectory. Given any
+/// algebra `A` with addition, negation,
+/// multiplication, and conjugation, the doubled algebra `Doubled<'A>`
+/// consists of pairs `(a, b)` with
+///
+///     (a, b) + (c, d) = (a + c, b + d)
+///     (a, b) · (c, d) = (a · c − d̄ · b, d · a + b · c̄)
+///     conj (a, b)    = (conj a, −b)
+///
+/// Applied iteratively:
+///
+///     ℝ → ℂ        — Real → Complex; loses total ordering
+///     ℂ → ℍ        — Complex → Quaternion; loses commutativity
+///     ℍ → 𝕆        — Quaternion → Octonion; loses associativity
+///     𝕆 → 𝕊        — Octonion → Sedenion; loses alternativity + division algebra
+///
+/// This is the substrate B-0623 names as the carrier of the "imaginary
+/// direction" in the cognitive boot sequence; Adinkras (B-0623 follow-up
+/// PRs) decorate this structure with colored-edge ECC information. The
+/// doubling primitive is the load-bearing object; specific levels
+/// (Complex, Quaternion, Octonion) are derived type aliases.
+///
+/// Implementation note: operations on `'A` are supplied via the
+/// `IAlgebra<'A>` dictionary rather than F# statically-resolved type
+/// parameters. The dictionary form makes the lift `IAlgebra<'A> →
+/// IAlgebra<Doubled<'A>>` directly expressible (the structural fact
+/// that motivates this module). SRTPs would require duplicated
+/// per-arity inline functions and lose the explicit "I doubled the
+/// algebra" surface that's the entire point.
+type Doubled<'A> = { Real: 'A; Imag: 'A }
+
+/// Algebra operations required for Cayley–Dickson doubling. A type
+/// `'A` qualifies as a (suitably-structured) algebra by providing
+/// zero, addition, negation, multiplication, and conjugation. The
+/// conjugation operation is the involutive antihomomorphism that
+/// makes the doubling well-defined — for ℝ it's the identity; for ℂ
+/// it's the standard complex conjugate; for ℍ and beyond it's defined
+/// recursively by the doubling formula above.
+type IAlgebra<'A> =
+    abstract Zero : 'A
+    abstract Add : 'A * 'A -> 'A
+    abstract Negate : 'A -> 'A
+    abstract Mul : 'A * 'A -> 'A
+    abstract Conj : 'A -> 'A
+
+[<RequireQualifiedAccess>]
+module Doubled =
+
+    /// Construct a doubled element from its real and imaginary parts.
+    let make (real: 'A) (imag: 'A) : Doubled<'A> = { Real = real; Imag = imag }
+
+    /// Lift an algebra on `'A` to an algebra on `Doubled<'A>`.
+    /// This IS the Cayley–Dickson construction; everything else
+    /// in this module is bookkeeping.
+    let algebra (inner: IAlgebra<'A>) : IAlgebra<Doubled<'A>> =
+        { new IAlgebra<Doubled<'A>> with
+            member _.Zero =
+                { Real = inner.Zero; Imag = inner.Zero }
+
+            member _.Add(x, y) =
+                { Real = inner.Add(x.Real, y.Real)
+                  Imag = inner.Add(x.Imag, y.Imag) }
+
+            member _.Negate x =
+                { Real = inner.Negate x.Real
+                  Imag = inner.Negate x.Imag }
+
+            // (a, b) · (c, d) = (a·c − d̄·b, d·a + b·c̄)
+            member _.Mul(x, y) =
+                let realPart =
+                    inner.Add(
+                        inner.Mul(x.Real, y.Real),
+                        inner.Negate(inner.Mul(inner.Conj y.Imag, x.Imag))
+                    )
+                let imagPart =
+                    inner.Add(
+                        inner.Mul(y.Imag, x.Real),
+                        inner.Mul(x.Imag, inner.Conj y.Real)
+                    )
+                { Real = realPart; Imag = imagPart }
+
+            // conj (a, b) = (conj a, −b)
+            member _.Conj x =
+                { Real = inner.Conj x.Real
+                  Imag = inner.Negate x.Imag } }
+
+
+/// Base case of the imaginary stack: real numbers as a degenerate
+/// algebra over themselves. Conjugation is the identity because ℝ
+/// has no imaginary part. Using `float` rather than an exact rational
+/// type intentionally — the stack is about structural properties of
+/// the doubling, not numerical precision; exact-arithmetic variants
+/// can be derived later by providing an `IAlgebra<Rational>` instead.
+[<RequireQualifiedAccess>]
+module Real =
+
+    let algebra : IAlgebra<float> =
+        { new IAlgebra<float> with
+            member _.Zero = 0.0
+            member _.Add(x, y) = x + y
+            member _.Negate x = -x
+            member _.Mul(x, y) = x * y
+            // ℝ-conjugation is identity (no imaginary part to flip).
+            member _.Conj x = x }
+
+
+/// Type aliases for the first few levels of the imaginary stack.
+/// `Complex` = ℝ doubled once; `Quaternion` = ℂ doubled; `Octonion` =
+/// ℍ doubled; `Sedenion` = 𝕆 doubled. The naming makes the imaginary
+/// stack's stratification readable at the type level, matching the
+/// trajectory requirement that category-theory structure remain visible
+/// in public type shapes.
+type Complex = Doubled<float>
+type Quaternion = Doubled<Complex>
+type Octonion = Doubled<Quaternion>
+type Sedenion = Doubled<Octonion>
+
+
+/// Pre-computed algebra instances at each level of the imaginary
+/// stack. Constructing these once and reusing them is correct because
+/// `IAlgebra<'A>` carries no state — it's a pure dictionary of
+/// operations. The instances are the structural fact "ℝ → ℂ → ℍ → 𝕆
+/// is the imaginary stack" made concrete.
+[<RequireQualifiedAccess>]
+module ImaginaryStack =
+
+    let complex : IAlgebra<Complex> = Doubled.algebra Real.algebra
+    let quaternion : IAlgebra<Quaternion> = Doubled.algebra complex
+    let octonion : IAlgebra<Octonion> = Doubled.algebra quaternion
+    let sedenion : IAlgebra<Sedenion> = Doubled.algebra octonion
diff --git a/src/Core/Core.fsproj b/src/Core/Core.fsproj
index d5c03706a8..3d15102ba6 100644
--- a/src/Core/Core.fsproj
+++ b/src/Core/Core.fsproj
@@ -13,6 +13,7 @@
   <ItemGroup>
     <Compile Include="AssemblyInfo.fs" />
     <Compile Include="Algebra.fs" />
+    <Compile Include="CayleyDickson.fs" />
     <Compile Include="Semiring.fs" />
     <Compile Include="Pool.fs" />
     <Compile Include="ZSet.fs" />
diff --git a/tests/Tests.FSharp/Algebra/CayleyDickson.Tests.fs b/tests/Tests.FSharp/Algebra/CayleyDickson.Tests.fs
new file mode 100644
index 0000000000..98a6a409de
--- /dev/null
+++ b/tests/Tests.FSharp/Algebra/CayleyDickson.Tests.fs
@@ -0,0 +1,198 @@
+module Zeta.Tests.Algebra.CayleyDicksonTests
+#nowarn "0893"
+
+/// Tests for `CayleyDickson` — the structural primitive underlying
+/// the imaginary stack (B-0623 PR1). Property structure mirrors the
+/// classical Cayley-Dickson loss pattern: each doubling step should
+/// preserve some algebraic invariants and lose specific others.
+///
+///   ℝ → ℂ        — addition stays Abelian; conjugation becomes
+///                  non-trivial; ordering is no longer total (we
+///                  don't test ordering loss since we never asserted
+///                  ordering at the ℝ level).
+///   ℂ → ℍ        — multiplication remains associative; loses
+///                  commutativity (i*j ≠ j*i).
+///   ℍ → 𝕆        — loses associativity; we exhibit a specific
+///                  triple (a, b, c) with (a·b)·c ≠ a·(b·c).
+///
+/// The shipping property "addition is associative + commutative at
+/// every level" is verified at ℂ, ℍ, 𝕆 since it should NEVER break.
+
+open FsUnit.Xunit
+open global.Xunit
+open Zeta.Core
+
+
+// ─── Helpers ──────────────────────────────────────────────────────────
+
+/// Approximate equality for floats — Cayley-Dickson at the float
+/// level inherits all the IEEE 754 quirks; tests use this threshold
+/// to keep them stable across platforms.
+let private approxEq (a: float) (b: float) =
+    abs (a - b) < 1e-9
+
+let private complexApproxEq (a: Complex) (b: Complex) =
+    approxEq a.Real b.Real && approxEq a.Imag b.Imag
+
+let private quaternionApproxEq (a: Quaternion) (b: Quaternion) =
+    complexApproxEq a.Real b.Real && complexApproxEq a.Imag b.Imag
+
+let private octonionApproxEq (a: Octonion) (b: Octonion) =
+    quaternionApproxEq a.Real b.Real && quaternionApproxEq a.Imag b.Imag
+
+
+// ─── Complex (ℂ) ──────────────────────────────────────────────────────
+// The first doubling. i² = −1 is the defining relation.
+
+[<Fact>]
+let ``Complex: i squared equals negative one`` () =
+    let alg = ImaginaryStack.complex
+    let i : Complex = Doubled.make 0.0 1.0
+    let result = alg.Mul(i, i)
+    let expected = Doubled.make -1.0 0.0
+    complexApproxEq result expected |> should be True
+
+
+[<Fact>]
+let ``Complex: addition is commutative`` () =
+    let alg = ImaginaryStack.complex
+    let a = Doubled.make 3.0 4.0
+    let b = Doubled.make -1.0 2.5
+    complexApproxEq (alg.Add(a, b)) (alg.Add(b, a)) |> should be True
+
+
+[<Fact>]
+let ``Complex: multiplication is commutative`` () =
+    // ℂ retains commutativity; it's quaternions that lose it.
+    let alg = ImaginaryStack.complex
+    let a = Doubled.make 1.5 -2.0
+    let b = Doubled.make 3.0 0.5
+    complexApproxEq (alg.Mul(a, b)) (alg.Mul(b, a)) |> should be True
+
+
+[<Fact>]
+let ``Complex: conjugation flips sign of imaginary part`` () =
+    let alg = ImaginaryStack.complex
+    let z = Doubled.make 2.0 -3.5
+    let conjZ = alg.Conj z
+    conjZ.Real |> should equal 2.0
+    conjZ.Imag |> should equal 3.5
+
+
+// ─── Quaternion (ℍ) ────────────────────────────────────────────────────
+// Second doubling. Loses commutativity but stays associative.
+
+[<Fact>]
+let ``Quaternion: i squared equals negative one`` () =
+    let alg = ImaginaryStack.quaternion
+    let zeroC : Complex = Doubled.make 0.0 0.0
+    let i : Quaternion = Doubled.make (Doubled.make 0.0 1.0) zeroC
+    let result = alg.Mul(i, i)
+    let expectedReal : Complex = Doubled.make -1.0 0.0
+    complexApproxEq result.Real expectedReal |> should be True
+    complexApproxEq result.Imag zeroC |> should be True
+
+
+[<Fact>]
+let ``Quaternion: multiplication loses commutativity (i*j != j*i)`` () =
+    let alg = ImaginaryStack.quaternion
+    // i = (i, 0) where the inner i is ℂ's i = (0, 1)
+    // j = (0, 1) where 1 is ℂ's (1, 0) and the embedding lifts it to imag
+    let zeroC : Complex = Doubled.make 0.0 0.0
+    let oneC : Complex = Doubled.make 1.0 0.0
+    let i : Quaternion = Doubled.make (Doubled.make 0.0 1.0) zeroC
+    let j : Quaternion = Doubled.make zeroC oneC
+    let ij = alg.Mul(i, j)
+    let ji = alg.Mul(j, i)
+    // In ℍ, ij = k and ji = −k, so ij ≠ ji.
+    quaternionApproxEq ij ji |> should be False
+
+
+[<Fact>]
+let ``Quaternion: multiplication is still associative`` () =
+    let alg = ImaginaryStack.quaternion
+    // Pick three non-trivial quaternions; verify (a·b)·c = a·(b·c).
+    let a : Quaternion = Doubled.make (Doubled.make 1.0 2.0) (Doubled.make 3.0 4.0)
+    let b : Quaternion = Doubled.make (Doubled.make 5.0 6.0) (Doubled.make 7.0 8.0)
+    let c : Quaternion = Doubled.make (Doubled.make 9.0 0.5) (Doubled.make -1.0 2.5)
+    let ab_c = alg.Mul(alg.Mul(a, b), c)
+    let a_bc = alg.Mul(a, alg.Mul(b, c))
+    quaternionApproxEq ab_c a_bc |> should be True
+
+
+[<Fact>]
+let ``Quaternion: addition stays commutative across the lift`` () =
+    let alg = ImaginaryStack.quaternion
+    let a : Quaternion = Doubled.make (Doubled.make 1.0 2.0) (Doubled.make 3.0 4.0)
+    let b : Quaternion = Doubled.make (Doubled.make -1.5 0.5) (Doubled.make 2.5 -3.5)
+    quaternionApproxEq (alg.Add(a, b)) (alg.Add(b, a)) |> should be True
+
+
+// ─── Octonion (𝕆) ──────────────────────────────────────────────────────
+// Third doubling. Loses associativity. Addition + commutativity-of-addition
+// still hold; multiplication is non-commutative AND non-associative.
+
+[<Fact>]
+let ``Octonion: addition stays commutative`` () =
+    let alg = ImaginaryStack.octonion
+    let mk a b c d e f g h : Octonion =
+        Doubled.make
+            (Doubled.make (Doubled.make a b) (Doubled.make c d))
+            (Doubled.make (Doubled.make e f) (Doubled.make g h))
+    let a = mk 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
+    let b = mk -1.5 0.5 2.5 -3.5 0.25 -0.75 1.25 -2.25
+    octonionApproxEq (alg.Add(a, b)) (alg.Add(b, a)) |> should be True
+
+
+[<Fact>]
+let ``Octonion: exhibits non-associativity for a specific triple`` () =
+    // Famous example: take three orthogonal imaginary units in 𝕆 that
+    // form an "associative triple" in ℍ but NOT in 𝕆. Here we use a
+    // simpler approach — pick three octonions and show (a·b)·c ≠ a·(b·c)
+    // numerically. The doubling formula guarantees this happens for
+    // generic non-trivial elements; we don't need to construct the
+    // textbook example.
+    let alg = ImaginaryStack.octonion
+    let mk a b c d e f g h : Octonion =
+        Doubled.make
+            (Doubled.make (Doubled.make a b) (Doubled.make c d))
+            (Doubled.make (Doubled.make e f) (Doubled.make g h))
+    let a = mk 1.0 0.0 0.0 0.0 0.0 1.0 0.0 0.0  // e_0 + e_5
+    let b = mk 0.0 0.0 1.0 0.0 0.0 0.0 0.0 1.0  // e_2 + e_7
+    let c = mk 0.0 1.0 0.0 0.0 0.0 0.0 1.0 0.0  // e_1 + e_6
+    let ab_c = alg.Mul(alg.Mul(a, b), c)
+    let a_bc = alg.Mul(a, alg.Mul(b, c))
+    octonionApproxEq ab_c a_bc |> should be False
+
+
+// ─── Structural / Zero / Identity ──────────────────────────────────────
+// Zero is the additive identity at every level; verify the lift
+// preserves this property.
+
+[<Fact>]
+let ``Zero is additive identity at every level`` () =
+    let c = ImaginaryStack.complex
+    let q = ImaginaryStack.quaternion
+    let o = ImaginaryStack.octonion
+    let cZero = c.Zero
+    let qZero = q.Zero
+    let oZero = o.Zero
+    let cVal = Doubled.make 3.0 -4.0
+    let qVal = Doubled.make cVal cZero
+    let oVal = Doubled.make qVal qZero
+    complexApproxEq (c.Add(cVal, cZero)) cVal |> should be True
+    quaternionApproxEq (q.Add(qVal, qZero)) qVal |> should be True
+    octonionApproxEq (o.Add(oVal, oZero)) oVal |> should be True
+
+
+[<Fact>]
+let ``Negation is additive inverse at every level`` () =
+    let c = ImaginaryStack.complex
+    let q = ImaginaryStack.quaternion
+    let o = ImaginaryStack.octonion
+    let cVal = Doubled.make 1.5 -2.5
+    let qVal = Doubled.make cVal (Doubled.make 0.5 0.25)
+    let oVal = Doubled.make qVal (Doubled.make (Doubled.make 1.0 1.0) (Doubled.make 1.0 1.0))
+    complexApproxEq (c.Add(cVal, c.Negate cVal)) c.Zero |> should be True
+    quaternionApproxEq (q.Add(qVal, q.Negate qVal)) q.Zero |> should be True
+    octonionApproxEq (o.Add(oVal, o.Negate oVal)) o.Zero |> should be True
diff --git a/tests/Tests.FSharp/Tests.FSharp.fsproj b/tests/Tests.FSharp/Tests.FSharp.fsproj
index 769209a75c..8dc68848fb 100644
--- a/tests/Tests.FSharp/Tests.FSharp.fsproj
+++ b/tests/Tests.FSharp/Tests.FSharp.fsproj
@@ -15,6 +15,7 @@
 
     <!-- Algebra/ -->
     <Compile Include="Algebra/Weight.Tests.fs" />
+    <Compile Include="Algebra/CayleyDickson.Tests.fs" />
     <Compile Include="Algebra/Units.Tests.fs" />
     <Compile Include="Algebra/ZSet.Tests.fs" />
     <Compile Include="Algebra/ZSet.Overflow.Tests.fs" />