diff --git a/theories/Core/Completeness/LogicalRelation.v b/theories/Core/Completeness/LogicalRelation.v
new file mode 100644
index 00000000..c3558135
--- /dev/null
+++ b/theories/Core/Completeness/LogicalRelation.v
@@ -0,0 +1,33 @@
+From Coq Require Import Relations.
+From Mcltt Require Import Base Domain Evaluation PER.
+Import Domain_Notations.
+
+Inductive rel_exp (R : relation domain) M p M' p' : Prop :=
+| mk_rel_exp : forall m m', {{ ⟦ M ⟧ p ↘ m }} -> {{ ⟦ M' ⟧ p' ↘ m' }} -> {{ Dom m ≈ m' ∈ R }} -> rel_exp R M p M' p'.
+#[global]
+Arguments mk_rel_exp {_ _ _ _ _}.
+
+Definition rel_exp_under_ctx Γ M M' A :=
+  exists env_rel (_ : {{ EF Γ ≈ Γ ∈ per_ctx_env ↘ env_rel }}) i
+     (elem_rel : forall {p p'} (equiva_p_p' : {{ Dom p ≈ p' ∈ env_rel }}), relation domain),
+  forall {p p'} (equiv_p_p' : {{ Dom p ≈ p' ∈ env_rel }}),
+    rel_typ i A p A p' (elem_rel equiv_p_p') /\ rel_exp (elem_rel equiv_p_p') M p M' p'.
+
+Definition valid_exp_under_ctx Γ M A := rel_exp_under_ctx Γ M M A.
+#[global]
+Arguments valid_exp_under_ctx _ _ _ /.
+
+Inductive rel_subst (R : relation env) σ p σ' p' : Prop :=
+| mk_rel_subst : forall o o', {{ ⟦ σ ⟧s p ↘ o }} -> {{ ⟦ σ' ⟧s p' ↘ o' }} -> {{ Dom o ≈ o' ∈ R }} -> rel_subst R σ p σ' p'.
+#[global]
+Arguments mk_rel_subst {_ _ _ _ _ _}.
+
+Definition rel_subst_under_ctx Γ σ σ' Δ :=
+  exists env_rel (_ : {{ EF Γ ≈ Γ ∈ per_ctx_env ↘ env_rel }})
+     env_rel' (_ : {{ EF Δ ≈ Δ ∈ per_ctx_env ↘ env_rel' }}),
+  forall {p p'} (equiv_p_p' : {{ Dom p ≈ p' ∈ env_rel }}),
+    rel_subst env_rel σ p σ' p'.
+
+Definition valid_subst_under_ctx Γ σ Δ := rel_subst_under_ctx Γ σ σ Δ.
+#[global]
+Arguments valid_subst_under_ctx _ _ _ /.
diff --git a/theories/_CoqProject b/theories/_CoqProject
index a78ff45f..fda4437f 100644
--- a/theories/_CoqProject
+++ b/theories/_CoqProject
@@ -4,6 +4,7 @@
 
 ./Core/Axioms.v
 ./Core/Base.v
+./Core/Completeness/LogicalRelation.v
 ./Core/Semantic/Domain.v
 ./Core/Semantic/Evaluation.v
 ./Core/Semantic/Evaluation/Definitions.v