diff --git a/Polyhedral.lean b/Polyhedral.lean
index ca634446..9db8e0c5 100644
--- a/Polyhedral.lean
+++ b/Polyhedral.lean
@@ -2,6 +2,7 @@ module  -- shake: keep-all --deprecated_module: ignore
 
 public import Polyhedral.Mathlib.Algebra.Group.Pointwise.SetLike.Basic
 public import Polyhedral.Mathlib.Algebra.Group.Pointwise.SetLike.Scalar
+public import Polyhedral.Mathlib.Algebra.Module.Lattice.Basic
 public import Polyhedral.Mathlib.Algebra.Module.LinearMap
 public import Polyhedral.Mathlib.Algebra.Module.Submodule.Basic
 public import Polyhedral.Mathlib.Algebra.Module.Submodule.Dual.Basic
diff --git a/Polyhedral/Mathlib/Algebra/Module/Lattice/Basic.lean b/Polyhedral/Mathlib/Algebra/Module/Lattice/Basic.lean
new file mode 100644
index 00000000..57127e13
--- /dev/null
+++ b/Polyhedral/Mathlib/Algebra/Module/Lattice/Basic.lean
@@ -0,0 +1,172 @@
+/-
+Copyright (c) 2026 Anouk Brose, Justus Springer. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Anouk Brose, Justus Springer
+-/
+module
+
+public import Mathlib.LinearAlgebra.Dimension.Localization
+public import Mathlib.LinearAlgebra.FiniteDimensional.Lemmas
+public import Mathlib.LinearAlgebra.FreeModule.PID
+public import Mathlib.Algebra.Module.Torsion.Free
+public import Mathlib.Algebra.Module.Lattice
+
+/-!
+# Lattices
+
+A generalization of mathlib's `Submodule.IsLattice` to make it useful for Polyhedral.
+This definition is used in the proof of Gordan's lemma, see
+`Polyhedral/Mathlib/Geometry/Convex/Cone/Pointed/Gordan.lean`.
+
+We (The Ehrhart group from the workshop) will work on upstreaming this into mathlib,
+to replace the existing definition. See Zulip discussion:
+
+https://leanprover.zulipchat.com/#narrow/channel/116395-maths/topic/Definitions.20of.20lattices/with/620077176
+
+-/
+
+@[expose] public section
+
+open Module
+open scoped Pointwise
+
+universe v w
+
+variable {R : Type*} [CommRing R]
+
+namespace Submodule
+
+/-- Let `K` be a `R`-algebra and `V` a `K`-module.
+A lattice is a `R`-submodule `M ≤ V` that is finitely generated such that every `R`-independent
+subset of `M` is `K`-linearly independent. Equivalently, the `R`-rank of `M` equals the
+`K`-dimension of its `K`-span, see `Submodule.isLattice'_iff_fg_and_finrank_eq`. -/
+class IsLattice' (A : outParam Type*) [CommRing A] [Algebra R A] {V : Type v} [AddCommMonoid V]
+    [Module R V] [Module A V] [IsScalarTower R A V] (M : Submodule R V) : Prop where
+  fg : M.FG
+  linearIndepOn : ∀ s ⊆ (M : Set V), LinearIndepOn R id s → LinearIndepOn A id s
+
+namespace IsLattice'
+
+section CommRing
+
+variable (A : Type*) [CommRing A] [Algebra R A]
+variable {V : Type v} [AddCommGroup V] [Module R V] [Module A V] [IsScalarTower R A V]
+variable (M : Submodule R V)
+
+/-- Any `R`-independent family of vectors in a lattice is `K`-linearly independent. -/
+theorem linearIndependent [IsLattice' A M] {ι : Type*} {v : ι → V}
+    (hv : ∀ i, v i ∈ M) (h : LinearIndependent R v) : LinearIndependent A v := by
+  cases subsingleton_or_nontrivial R
+  · have := (algebraMap R A).codomain_trivial
+    exact linearIndependent_of_subsingleton
+  exact (linearIndepOn_id_range_iff h.injective).mp <|
+    IsLattice'.linearIndepOn _ (Set.range_subset_iff.mpr hv) h.linearIndepOn_id
+
+/-- Any basis of an `R`-lattice in `V` is `K`-linearly independent. -/
+theorem basis_linearIndependent {I : Type w} {M : Submodule R V}
+    [IsLattice' A M] (b : Basis I R M) : LinearIndependent A (fun i ↦ (b i).val) :=
+  IsLattice'.linearIndependent A M (fun i ↦ (b i).2)
+    (b.linearIndependent.map' M.subtype (Submodule.ker_subtype _))
+
+/-- Any `R`-lattice is finite. -/
+instance finite [IsLattice' A M] : Module.Finite R M := by
+  rw [Module.Finite.iff_fg]
+  exact IsLattice'.fg
+
+theorem mono_of_fg {M N : Submodule R V} (hle : M ≤ N) (hfg : M.FG) [IsLattice' A N] :
+    IsLattice' A M where
+  fg := hfg
+  linearIndepOn s hs hli := linearIndepOn s (hs.trans hle) hli
+
+/-- Over a Noetherian ring, any submodule of a lattice is a lattice. -/
+theorem mono [IsNoetherianRing R] {M N : Submodule R V} (hle : M ≤ N) [IsLattice' A N] :
+    IsLattice' A M :=
+  mono_of_fg A hle (isNoetherian_submodule.mp inferInstance M hle)
+
+/-- Over a Noetherian ring, the intersection of two lattices is a lattice. -/
+instance inf [IsNoetherianRing R] (M N : Submodule R V) [IsLattice' A M] [IsLattice' A N] :
+    IsLattice' A (M ⊓ N) :=
+  mono A inf_le_left
+
+end CommRing
+
+section Field
+
+variable (K : Type*) [Field K] [Algebra R K] [FaithfulSMul R K]
+variable {V : Type v} [AddCommGroup V] [Module R V] [Module K V] [IsScalarTower R K V]
+variable (M : Submodule R V)
+
+-- This has been merged into mathlib: #43297
+theorem rank_span_le_rank : Module.rank K (span K (M : Set V)) ≤ Module.rank R M := by
+  have : Nontrivial R := (algebraMap R K).domain_nontrivial
+  obtain ⟨b, hbM, hbspan, hbli⟩ := exists_linearIndependent K (M : Set V)
+  rw [← hbspan, rank_span_set hbli]
+  have hb : LinearIndependent R (fun x : b ↦ (⟨x.1, hbM x.2⟩ : M)) :=
+    LinearIndependent.of_comp M.subtype (hbli.restrict_scalars' R)
+  exact hb.cardinal_le_rank
+
+/-- The `R`-rank of a lattice equals the `K`-dimension of its `K`-span. -/
+theorem rank_eq_rank_span [IsLattice' K M] :
+    Module.rank R M = Module.rank K (span K (M : Set V)) := by
+  -- AI slop, needs refactor
+  refine le_antisymm ?_ (rank_span_le_rank K M)
+  rw [Module.rank]
+  apply ciSup_le'
+  intro ⟨s, hs⟩
+  have hR : LinearIndependent R (fun x : s ↦ ((x : M) : V)) :=
+    hs.map' M.subtype (Submodule.ker_subtype _)
+  have hK : LinearIndependent K (fun x : s ↦ ((x : M) : V)) :=
+    IsLattice'.linearIndependent K M (fun x ↦ (x : M).2) hR
+  have hmem : ∀ x : s, ((x : M) : V) ∈ span K (M : Set V) := fun x ↦ subset_span (x : M).2
+  have hW : LinearIndependent K
+      (fun x : s ↦ (⟨((x : M) : V), hmem x⟩ : span K (M : Set V))) :=
+    LinearIndependent.of_comp (span K (M : Set V)).subtype hK
+  exact hW.cardinal_le_rank
+
+/-- The `R`-rank of a lattice equals the `K`-dimension of its `K`-span. -/
+theorem finrank_eq_finrank_span [IsLattice' K M] :
+    finrank R M = finrank K (span K (M : Set V)) :=
+  congrArg Cardinal.toNat (rank_eq_rank_span K M)
+
+theorem _root_.Submodule.isLattice'_iff_fg_and_finrank_eq :
+    IsLattice' K M ↔ M.FG ∧ finrank R M = finrank K (span K (M : Set V)) := by
+  -- Steinitz argument?
+  sorry
+
+section IsPrincipalIdealRing
+
+variable [IsDomain R] [IsPrincipalIdealRing R]
+
+/-- Any lattice over a PID is a free `R`-module. -/
+instance free (M : Submodule R V) [IsLattice' K M] : Module.Free R M := by
+  have := Module.IsTorsionFree.trans_faithfulSMul R K V
+  -- any torsion free finite module over a PID is free
+  infer_instance
+
+/-- A lattice over a PID has a basis that's `K`-linearly independent. -/
+theorem exists_basis_linearIndependent (M : Submodule R V) [IsLattice' K M] :
+    ∃ (I : Type v) (b : Basis I R M), LinearIndependent K (fun i ↦ (b i).val) := by
+  obtain ⟨I, b⟩ := Module.Free.exists_basis R M
+  exact ⟨I, b, basis_linearIndependent K b⟩
+
+end IsPrincipalIdealRing
+
+section IsFractionRing
+
+variable (K : Type*) [Field K] [Algebra R K] [IsFractionRing R K]
+variable {V : Type*} [AddCommGroup V] [Module R V] [Module K V] [IsScalarTower R K V]
+
+/-- If `K` is the field of fractions of `R`, any finitely generated `R`-submodule of `V`
+is a lattice. -/
+theorem of_fg {M : Submodule R V} (hM : M.FG) : IsLattice' K M where
+  fg := hM
+  linearIndepOn v hvM hv := by
+    sorry
+
+end IsFractionRing
+
+end Field
+
+end IsLattice'
+
+end Submodule