diff --git a/OpenGALib/Manifold.lean b/OpenGALib/Manifold.lean
index f63f740..3b671a6 100644
--- a/OpenGALib/Manifold.lean
+++ b/OpenGALib/Manifold.lean
@@ -1,4 +1,5 @@
 import OpenGALib.Manifold.Charts.CoordinateBall
+import OpenGALib.Manifold.Charts.PrecompactBasis
 
 /-!
 # Manifold
diff --git a/OpenGALib/Manifold/Charts/CoordinateBall.lean b/OpenGALib/Manifold/Charts/CoordinateBall.lean
index 56de54a..29ef3ef 100644
--- a/OpenGALib/Manifold/Charts/CoordinateBall.lean
+++ b/OpenGALib/Manifold/Charts/CoordinateBall.lean
@@ -1,6 +1,5 @@
 import Mathlib.Analysis.InnerProductSpace.PiL2
 import Mathlib.Geometry.Manifold.ChartedSpace
-import Mathlib.Geometry.Manifold.ContMDiff.Atlas
 import Mathlib.Geometry.Manifold.Instances.Real
 import Mathlib.Topology.Bases
 import Mathlib.Topology.Separation.Basic
diff --git a/OpenGALib/Manifold/Charts/PrecompactBasis.lean b/OpenGALib/Manifold/Charts/PrecompactBasis.lean
new file mode 100644
index 0000000..2b03792
--- /dev/null
+++ b/OpenGALib/Manifold/Charts/PrecompactBasis.lean
@@ -0,0 +1,256 @@
+import Mathlib.Analysis.InnerProductSpace.PiL2
+import Mathlib.Geometry.Manifold.ChartedSpace
+import Mathlib.Geometry.Manifold.Instances.Real
+import Mathlib.Topology.Bases
+import Mathlib.Topology.Compactness.Paracompact
+import Mathlib.Topology.Compactness.SigmaCompact
+import Mathlib.Topology.Separation.Basic
+import Mathlib.Topology.Sets.OpenCover
+import OpenGALib.Manifold.Charts.CoordinateBall
+
+/-!
+# Precompact coordinate balls and locally finite refinements
+
+Covering infrastructure beneath partitions of unity and exhaustions. A
+*precompact coordinate ball* is the source of a coordinate-ball chart with
+compact closure; on a topological `n`-manifold (Hausdorff, second-countable,
+charted over `EuclideanSpace ℝ (Fin n)`) these form a countable basis, and
+together with paracompactness every open cover admits a countable, locally
+finite refinement by precompact coordinate balls subordinate to it.
+
+The development is layered: the basis results need only the ambient topology
+and a chart; the refinement adds paracompactness, factored through an abstract
+basis-refinement lemma (`exists_countable_locallyFinite_refinement_of_isTopologicalBasis`)
+reusable for any topological basis.
+
+## Main definitions
+
+* `Manifold.IsPrecompactCoordinateBall` — source of a coordinate-ball chart with compact closure.
+
+## Main results
+
+* `Manifold.IsPrecompactCoordinateBall.isOpen` — a precompact coordinate ball is open.
+* `Manifold.isTopologicalBasis_isPrecompactCoordinateBall` — precompact coordinate balls form a basis.
+* `Manifold.exists_countable_precompact_coordinate_ball_basis` — that basis can be taken countable.
+* `Manifold.paracompactSpace_of_manifold` — a topological manifold is paracompact.
+* `Manifold.exists_countable_locally_finite_precompact_coordinate_ball_refinement` —
+  every open cover has a countable locally finite refinement by precompact coordinate balls.
+
+Provenance: SmoothManifoldsLee a5f308c — Lemma_1_10, Theorem_1_15.
+-/
+
+open Set TopologicalSpace
+open scoped Topology
+
+universe u v
+
+namespace Manifold
+
+section Basis
+
+variable {n : ℕ} {M : Type u} [TopologicalSpace M]
+
+/-- **Math.** A set `s ⊆ M` is a *precompact coordinate ball*: it is the source of
+some coordinate-ball chart and its closure is compact. -/
+def IsPrecompactCoordinateBall (n : ℕ) (s : Set M) : Prop :=
+  (∃ e : OpenPartialHomeomorph M (EuclideanSpace ℝ (Fin n)),
+    e.source = s ∧ e.IsCoordinateBall) ∧
+      IsCompact (closure s)
+
+/-- **Math.** A precompact coordinate ball is open. -/
+theorem IsPrecompactCoordinateBall.isOpen {s : Set M}
+    (hs : IsPrecompactCoordinateBall n s) : IsOpen s := by
+  rcases hs.1 with ⟨e, rfl, -⟩
+  simpa using e.open_source
+
+/-- **Math.** A precompact coordinate ball has compact closure. -/
+theorem IsPrecompactCoordinateBall.isCompact_closure {s : Set M}
+    (hs : IsPrecompactCoordinateBall n s) : IsCompact (closure s) :=
+  hs.2
+
+variable [T2Space M] [SecondCountableTopology M]
+  [ChartedSpace (EuclideanSpace ℝ (Fin n)) M]
+
+omit [SecondCountableTopology M] [ChartedSpace (EuclideanSpace ℝ (Fin n)) M] in
+/-- **Math.** Restricting a chart to a Euclidean open ball whose closed ball is
+compactly contained in the chart target produces a precompact coordinate ball in
+`M`. -/
+theorem isPrecompactCoordinateBall_chart_preimage_metric_ball
+    (e : OpenPartialHomeomorph M (EuclideanSpace ℝ (Fin n))) {c : EuclideanSpace ℝ (Fin n)}
+    {r : ℝ} (hr : 0 < r) (hclosed : Metric.closedBall c r ⊆ e.target) :
+    IsPrecompactCoordinateBall n (e.symm '' Metric.ball c r) := by
+  let e' := e.trans (OpenPartialHomeomorph.ofSet (Metric.ball c r) Metric.isOpen_ball)
+  have hball_target : Metric.ball c r ⊆ e.target := by
+    exact (Metric.ball_subset_closedBall : Metric.ball c r ⊆ Metric.closedBall c r).trans hclosed
+  have hsource : e'.source = e.symm '' Metric.ball c r := by
+    -- The restricted chart is defined exactly on the inverse image of the chosen ball.
+    dsimp [e']
+    exact (e.symm_image_eq_source_inter_preimage hball_target).symm
+  have htarget : e'.target = Metric.ball c r := by
+    -- On the target side, the restriction keeps precisely the chosen Euclidean ball.
+    dsimp [e']
+    exact inter_eq_left.2 hball_target
+  have hcompactCarrier : IsCompact (e.symm '' Metric.closedBall c r) := by
+    -- The closed ball is compact in Euclidean space, and the inverse chart is continuous on it.
+    exact (isCompact_closedBall (x := c) (r := r)).image_of_continuousOn
+      (e.continuousOn_symm.mono hclosed)
+  have hclosureCompact : IsCompact (closure (e.symm '' Metric.ball c r)) := by
+    -- The closure stays inside the compact pullback of the closed Euclidean ball.
+    exact hcompactCarrier.closure_of_subset
+      (Set.image_mono (Metric.ball_subset_closedBall : Metric.ball c r ⊆ Metric.closedBall c r))
+  refine ⟨?_, hclosureCompact⟩
+  refine ⟨e', hsource, ?_⟩
+  exact OpenPartialHomeomorph.isCoordinateBall_of_target_eq_ball e' c r hr htarget
+
+omit [SecondCountableTopology M] in
+/-- **Math.** Every point of an open set lies in a precompact coordinate ball
+contained in that open set. -/
+theorem exists_isPrecompactCoordinateBall_subset_of_mem_open {x : M}
+    {u : Set M} (hx : x ∈ u) (hu : IsOpen u) :
+    ∃ s : Set M, IsPrecompactCoordinateBall n s ∧ x ∈ s ∧ s ⊆ u := by
+  let e := chartAt (EuclideanSpace ℝ (Fin n)) x
+  let y : EuclideanSpace ℝ (Fin n) := e x
+  let w : Set (EuclideanSpace ℝ (Fin n)) := e '' (e.source ∩ u)
+  have hxsource : x ∈ e.source := mem_chart_source _ x
+  have hyw : y ∈ w := by
+    refine ⟨x, ⟨hxsource, hx⟩, rfl⟩
+  have hw_open : IsOpen w := by
+    -- The image of the open neighborhood inside the chart source is open in Euclidean space.
+    simpa [w] using e.isOpen_image_source_inter hu
+  obtain ⟨r, hr, hclosed⟩ :
+      ∃ r : ℝ, 0 < r ∧ Metric.closedBall y r ⊆ w := by
+    -- Choose a small closed Euclidean ball around the charted point inside that open image.
+    exact Metric.nhds_basis_closedBall.mem_iff.1 (hw_open.mem_nhds hyw)
+  have hw_target : w ⊆ e.target := by
+    -- The chart image of any subset of the source stays in the chart target.
+    simpa [w] using (Set.image_mono (show e.source ∩ u ⊆ e.source from inter_subset_left)).trans
+      e.image_source_eq_target.subset
+  let s : Set M := e.symm '' Metric.ball y r
+  have hs_precompact : IsPrecompactCoordinateBall n s := by
+    -- Pull back the chosen Euclidean ball through the chart.
+    simpa [s, y] using isPrecompactCoordinateBall_chart_preimage_metric_ball
+      (n := n) e hr (hclosed.trans hw_target)
+  have hxs : x ∈ s := by
+    -- The charted point lies in every positive-radius ball centered at itself.
+    refine ⟨y, Metric.mem_ball_self hr, ?_⟩
+    simpa [y] using e.left_inv hxsource
+  have hs_subset_source_u : s ⊆ e.source ∩ u := by
+    -- The pulled-back ball lies inside the original open neighborhood because its image does.
+    have hball_w : Metric.ball y r ⊆ w := by
+      exact (Metric.ball_subset_closedBall : Metric.ball y r ⊆ Metric.closedBall y r).trans hclosed
+    change e.symm '' Metric.ball y r ⊆ e.source ∩ u
+    refine (Set.image_mono hball_w).trans ?_
+    have himage : e.symm '' w = e.source ∩ u := by
+      simpa [w] using e.symm_image_image_of_subset_source
+        (s := e.source ∩ u) (show e.source ∩ u ⊆ e.source from inter_subset_left)
+    exact himage.subset
+  exact ⟨s, hs_precompact, hxs, fun z hz ↦ (hs_subset_source_u hz).2⟩
+
+omit [SecondCountableTopology M] in
+/-- **Math.** Precompact coordinate balls form a topological basis on a topological
+manifold. -/
+theorem isTopologicalBasis_isPrecompactCoordinateBall :
+    IsTopologicalBasis { s : Set M | IsPrecompactCoordinateBall n s } := by
+  -- The local chart construction gives a basis element inside every open neighborhood.
+  refine isTopologicalBasis_of_isOpen_of_nhds ?_ ?_
+  · intro s hs
+    exact hs.isOpen
+  · intro x u hx hu
+    obtain ⟨s, hs, hxs, hsu⟩ := exists_isPrecompactCoordinateBall_subset_of_mem_open
+      (n := n) hx hu
+    exact ⟨s, hs, hxs, hsu⟩
+
+/-- **Math.** Lemma 1.10: every topological manifold has a *countable* topological
+basis consisting of precompact coordinate balls. -/
+theorem exists_countable_precompact_coordinate_ball_basis :
+    ∃ b : Set (Set M),
+      b.Countable ∧ IsTopologicalBasis b ∧ ∀ s ∈ b, IsPrecompactCoordinateBall n s := by
+  let B : Set (Set M) := { s : Set M | IsPrecompactCoordinateBall n s }
+  have hB : IsTopologicalBasis B := isTopologicalBasis_isPrecompactCoordinateBall (n := n)
+  -- Once the local basis is available, second countability lets us thin it to a countable subbasis.
+  obtain ⟨b, hbB, hbCountable, hbBasis⟩ := hB.exists_countable
+  exact ⟨b, hbCountable, hbBasis, fun s hs ↦ hbB hs⟩
+
+end Basis
+
+section Refinement
+
+variable {n : ℕ} {M : Type u} [TopologicalSpace M] [T2Space M]
+  [SecondCountableTopology M] [ChartedSpace (EuclideanSpace ℝ (Fin n)) M]
+
+include n in
+/-- **Math.** A topological manifold is paracompact. -/
+theorem paracompactSpace_of_manifold : ParacompactSpace M := by
+  letI : LocallyCompactSpace M :=
+    ChartedSpace.locallyCompactSpace (EuclideanSpace ℝ (Fin n)) M
+  letI : SigmaCompactSpace M := sigmaCompactSpace_of_locallyCompact_secondCountable
+  infer_instance
+
+/-- **Math.** Companion bridge: given an open cover of a locally compact,
+sigma-compact, Hausdorff space and any basis for its topology, there is a countable
+locally finite refinement by basis elements subordinate to the cover. -/
+theorem exists_countable_locallyFinite_refinement_of_isTopologicalBasis {X : Type u}
+    [TopologicalSpace X] [LocallyCompactSpace X] [SigmaCompactSpace X] [T2Space X]
+    {ι : Type v} {U : ι → Opens X}
+    (hU : IsOpenCover U) {B : Set (Set X)} (hB : IsTopologicalBasis B) :
+    ∃ (κ : Type u), Countable κ ∧ ∃ V : κ → Opens X,
+      IsOpenCover V ∧ LocallyFinite (fun j ↦ (V j : Set X)) ∧
+        (∀ j, (V j : Set X) ∈ B) ∧
+          (∀ j, ∃ i, (V j : Set X) ⊆ U i) := by
+  -- Choose, at each point, one member of the original cover that contains that point.
+  choose i hi using fun x : X ↦ hU.exists_mem_nhds x
+  have hBU :
+      ∀ x : X,
+        (𝓝 x).HasBasis
+          (fun s : Set X ↦ s ∈ B ∧ x ∈ s ∧ s ⊆ U (i x))
+          id := by
+    intro x
+    let hxB : (𝓝 x).HasBasis (fun s : Set X ↦ s ∈ B ∧ x ∈ s) id := hB.nhds_hasBasis
+    simpa [and_assoc] using hxB.restrict_subset (hi x)
+  -- Feed the subordinate neighborhood bases into the standard locally finite refinement theorem.
+  rcases refinement_of_locallyCompact_sigmaCompact_of_nhds_basis hBU with
+    ⟨κ, c, W, hW_basis, hW_cover, hW_locallyFinite⟩
+  have hκ : Countable κ := by
+    simpa [Set.countable_univ_iff] using
+      hW_locallyFinite.countable_univ fun j ↦ ⟨c j, (hW_basis j).2.1⟩
+  refine ⟨κ, hκ, ?_⟩
+  refine ⟨fun j ↦ ⟨W j, hB.isOpen (hW_basis j).1⟩, ?_, hW_locallyFinite, ?_, ?_⟩
+  · exact IsOpenCover.of_sets (fun j ↦ hB.isOpen (hW_basis j).1) hW_cover
+  · exact fun j ↦ (hW_basis j).1
+  · exact fun j ↦ ⟨i (c j), (hW_basis j).2.2⟩
+
+/-- **Math.** Theorem 1.15: given an open cover of a topological manifold, there is a
+countable locally finite refinement by precompact coordinate balls subordinate to
+the cover. -/
+theorem exists_countable_locally_finite_precompact_coordinate_ball_refinement
+    {ι : Type v} {U : ι → Opens M} (hU : IsOpenCover U) :
+    ∃ (κ : Type u), Countable κ ∧ ∃ V : κ → Opens M,
+      IsOpenCover V ∧ LocallyFinite (fun j ↦ (V j : Set M)) ∧
+        (∀ j, IsPrecompactCoordinateBall n (V j : Set M)) ∧
+          (∀ j, ∃ i, (V j : Set M) ⊆ U i) := by
+  -- Start from the countable basis of precompact coordinate balls supplied earlier.
+  have hBasis :
+      ∃ B : Set (Set M),
+        B.Countable ∧ IsTopologicalBasis B ∧ ∀ s ∈ B, IsPrecompactCoordinateBall n s :=
+    exists_countable_precompact_coordinate_ball_basis
+  rcases hBasis with ⟨B, -, hB_basis, hB_precompact⟩
+  -- Apply the abstract refinement theorem to this specific basis.
+  have hRefinement :
+      ∃ (κ : Type u), Countable κ ∧ ∃ V : κ → Opens M,
+        IsOpenCover V ∧ LocallyFinite (fun j ↦ (V j : Set M)) ∧
+          (∀ j, (V j : Set M) ∈ B) ∧
+            (∀ j, ∃ i, (V j : Set M) ⊆ U i) :=
+    by
+      letI : LocallyCompactSpace M :=
+        ChartedSpace.locallyCompactSpace (EuclideanSpace ℝ (Fin n)) M
+      letI : SigmaCompactSpace M := sigmaCompactSpace_of_locallyCompact_secondCountable
+      exact exists_countable_locallyFinite_refinement_of_isTopologicalBasis hU hB_basis
+  rcases hRefinement with
+    ⟨κ, hκ, V, hV_cover, hV_locallyFinite, hV_mem, hV_subordinate⟩
+  refine ⟨κ, hκ, V, hV_cover, hV_locallyFinite, ?_, hV_subordinate⟩
+  intro j
+  exact hB_precompact _ (hV_mem j)
+
+end Refinement
+
+end Manifold
diff --git a/docs/log/2026-06-12-smooth-manifolds-lee/WORKLIST.md b/docs/log/2026-06-12-smooth-manifolds-lee/WORKLIST.md
index 80624dc..5a0f06d 100644
--- a/docs/log/2026-06-12-smooth-manifolds-lee/WORKLIST.md
+++ b/docs/log/2026-06-12-smooth-manifolds-lee/WORKLIST.md
@@ -115,11 +115,11 @@ strict match to the table above). Actual status takes precedence:
 | PR | Branch | Content | Status |
 |---|---|---|---|
 | #65 | `port/coordinate-ball` | coordinate-ball chart predicates | **merged to develop** (docstrings house-style-revised in db767b1, serves as the exemplar) |
-| #66 | `port/precompact-basis` | precompact coordinate-ball countable basis + locally-finite refinement | open, pending feedback: retarget base to develop, revise docstrings |
-| #67 | `port/charted-space-core` | manifold from a chart-family core | open, same |
-| #68 | `port/exhaustion-cutoff` | exhaustion functions + local smoothness + partition-of-unity ⇒ paracompact | open, same |
-| #69 | `port/curve-velocity-mfderiv` | curve velocity + mfderiv (has snake_case defs to rename) | open, same |
-| #70 | `port/coordinate-components` | coordinate tangent components (has snake_case defs; inlines `tangent_coordinates_change`) | open, same |
+| #66 | `port/precompact-basis` | precompact coordinate-ball countable basis + locally-finite refinement | **merged to develop** (cherry-pick 498d57e, docstrings house-style-revised; second exemplar) |
+| #67 | `port/charted-space-core` | manifold from a chart-family core | open, feedback posted |
+| #68 | `port/exhaustion-cutoff` | exhaustion functions + local smoothness + partition-of-unity ⇒ paracompact | open, feedback posted |
+| #69 | `port/curve-velocity-mfderiv` | curve velocity + mfderiv (has snake_case defs to rename) | open, feedback posted |
+| #70 | `port/coordinate-components` | coordinate tangent components (has snake_case defs; inlines `tangent_coordinates_change`) | open, feedback posted |
 
 Common feedback (pointing at merged #65 as the exemplar): ① docstring house
 style (single Math tags / architecture narrative / provenance footer); ②