diff --git a/Mathlib.lean b/Mathlib.lean
index 3a6d06bb68a207..65b9583af4f6ac 100644
--- a/Mathlib.lean
+++ b/Mathlib.lean
@@ -4044,6 +4044,7 @@ public import Mathlib.Geometry.Euclidean.Sphere.Tangent
 public import Mathlib.Geometry.Euclidean.Triangle
 public import Mathlib.Geometry.Group.Growth.LinearLowerBound
 public import Mathlib.Geometry.Group.Growth.QuotientInter
+public import Mathlib.Geometry.Group.Marking
 public import Mathlib.Geometry.Manifold.Algebra.LeftInvariantDerivation
 public import Mathlib.Geometry.Manifold.Algebra.LieGroup
 public import Mathlib.Geometry.Manifold.Algebra.Monoid
diff --git a/Mathlib/Geometry/Group/Marking.lean b/Mathlib/Geometry/Group/Marking.lean
new file mode 100644
index 00000000000000..daeee3e4ed101f
--- /dev/null
+++ b/Mathlib/Geometry/Group/Marking.lean
@@ -0,0 +1,172 @@
+/-
+Copyright (c) 2023 Yaël Dillies. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Yaël Dillies
+-/
+module
+
+public import Mathlib.Analysis.Normed.Group.Quotient
+public import Mathlib.GroupTheory.FreeGroup.Reduce
+
+/-!
+# Marked groups
+
+This file defines group markings and induces a norm on marked groups.
+
+## Main declarations
+
+* `GroupMarking G S`: Marking of the group `G` by a type `S`, namely a surjective monoid
+  homomorphism `FreeGroup S →* G`.
+* `MarkedGroup`: If `m : GroupMarking G S`, then `MarkedGroup m` is a type synonym for `G`
+  endowed with the metric coming from `m`.
+* `MarkedGroup.instNormedGroup`: A marked group is normed by the word metric of the marking.
+-/
+
+open Function List Nat
+
+variable {G S : Type*} [Group G]
+
+/-- A marking of an additive group is a generating family of elements. -/
+structure AddGroupMarking (G S : Type*) [AddGroup G] extends FreeAddGroup S →+ G where
+  toFun_surjective : Surjective toFun
+
+/-- A marking of a group is a generating family of elements. -/
+@[to_additive]
+structure GroupMarking (G S : Type*) [Group G] extends FreeGroup S →* G where
+  toFun_surjective : Surjective toFun
+
+/-- Reinterpret a marking of `G` by `S` as an additive monoid homomorphism `FreeAddGroup S →+ G`. -/
+add_decl_doc AddGroupMarking.toAddMonoidHom
+
+/-- Reinterpret a marking of `G` by `S` as a monoid homomorphism `FreeGroup S →+ G`. -/
+add_decl_doc GroupMarking.toMonoidHom
+
+namespace GroupMarking
+
+@[to_additive]
+instance instFunLike : FunLike (GroupMarking G S) (FreeGroup S) G where
+  coe f := f.toFun
+  coe_injective' := by rintro ⟨⟨⟨_, _⟩, _⟩, _⟩; congr!
+
+@[to_additive]
+instance instMonoidHomClass : MonoidHomClass (GroupMarking G S) (FreeGroup S) G where
+  map_mul f := f.map_mul'
+  map_one f := f.map_one'
+
+@[to_additive]
+lemma coe_surjective (m : GroupMarking G S) : Surjective m := m.toFun_surjective
+
+end GroupMarking
+
+/-- The trivial group marking. -/
+@[to_additive "The trivial additive group marking."]
+def GroupMarking.refl : GroupMarking G G :=
+  { FreeGroup.lift id with toFun_surjective := fun x => ⟨FreeGroup.of x, FreeGroup.lift.of⟩ }
+
+@[to_additive] instance : Inhabited (GroupMarking G G) := ⟨.refl⟩
+
+variable {m : GroupMarking G S}
+
+set_option linter.unusedVariables false in
+/-- A type synonym of `G`, tagged with a group marking. -/
+@[to_additive (attr := nolint unusedArguments)
+"A type synonym of `G`, tagged with an additive group marking."]
+def MarkedGroup (m : GroupMarking G S) : Type _ := G
+
+@[to_additive] instance MarkedGroup.instGroup : Group (MarkedGroup m) := ‹Group G›
+
+/-- "Identity" isomorphism between `G` and a group marking of it. -/
+@[to_additive "\"Identity\" isomorphism between `G` and an additive group marking of it."]
+def toMarkedGroup : G ≃* MarkedGroup m := .refl _
+
+/-- "Identity" isomorphism between a group marking of `G` and itself. -/
+@[to_additive "\"Identity\" isomorphism between an additive group marking of `G` and itself."]
+def ofMarkedGroup : MarkedGroup m ≃* G := .refl _
+
+@[to_additive (attr := simp)]
+lemma toMarkedGroup_symm_eq : (toMarkedGroup : G ≃* MarkedGroup m).symm = ofMarkedGroup := rfl
+
+@[to_additive (attr := simp)]
+lemma ofMarkedGroup_symm_eq : (ofMarkedGroup : MarkedGroup m ≃* G).symm = toMarkedGroup := rfl
+
+@[to_additive (attr := simp)]
+lemma toMarkedGroup_ofMarkedGroup (a) : toMarkedGroup (ofMarkedGroup (a : MarkedGroup m)) = a := rfl
+
+@[to_additive (attr := simp)]
+lemma ofMarkedGroup_toMarkedGroup (a) : ofMarkedGroup (toMarkedGroup a : MarkedGroup m) = a := rfl
+
+@[to_additive]
+lemma toMarkedGroup_inj {a b} : (toMarkedGroup a : MarkedGroup m) = toMarkedGroup b ↔ a = b := .rfl
+
+@[to_additive]
+lemma ofMarkedGroup_inj {a b : MarkedGroup m} : ofMarkedGroup a = ofMarkedGroup b ↔ a = b := .rfl
+
+variable (α : Type*)
+
+@[to_additive]
+instance MarkedGroup.instInhabited [Inhabited G] : Inhabited (MarkedGroup m) := ‹Inhabited G›
+
+@[to_additive]
+instance MarkedGroup.instSmul [SMul G α] : SMul (MarkedGroup m) α := ‹SMul G α›
+
+@[to_additive]
+instance MarkedGroup.instMulAction [MulAction G α] : MulAction (MarkedGroup m) α := ‹MulAction G α›
+
+@[to_additive (attr := simp)]
+lemma toMarkedGroup_smul (g : G) (x : α) [SMul G α] :
+    (toMarkedGroup g : MarkedGroup m) • x = g • x := rfl
+
+@[to_additive (attr := simp)]
+lemma ofMarkedGroup_smul (g : MarkedGroup m) (x : α) [SMul G α] : ofMarkedGroup g • x = g • x := rfl
+
+/-- A marked group is equivalent to a quotient of the free group by a normal subgroup. -/
+@[simps! apply]
+noncomputable def quotientEquivMarkedGroup : FreeGroup S ⧸ m.toMonoidHom.ker ≃* MarkedGroup m :=
+  QuotientGroup.quotientKerEquivOfSurjective _ m.coe_surjective
+
+@[to_additive]
+private lemma mul_aux [DecidableEq S] (x : MarkedGroup m) :
+    ∃ (n : _) (l : FreeGroup S), toMarkedGroup (m l) = x ∧ l.toWord.length ≤ n := by
+  classical
+  obtain ⟨l, rfl⟩ := m.coe_surjective x
+  exact ⟨_, _, rfl, le_rfl⟩
+
+@[to_additive]
+private lemma mul_aux' [DecidableEq S] (x : MarkedGroup m) :
+    ∃ (n : _) (l : FreeGroup S), toMarkedGroup (m l) = x ∧ l.toWord.length = n := by
+  classical
+  obtain ⟨l, rfl⟩ := m.coe_surjective x
+  exact ⟨_, _, rfl, rfl⟩
+
+@[to_additive]
+private lemma find_mul_aux [DecidableEq S] (x : MarkedGroup m)
+    [DecidablePred fun n ↦ ∃ l, toMarkedGroup (m l) = x ∧ l.toWord.length ≤ n]
+    [DecidablePred fun n ↦ ∃ l, toMarkedGroup (m l) = x ∧ l.toWord.length = n] :
+    Nat.find (mul_aux x) = Nat.find (mul_aux' x) := by
+  classical
+  exact _root_.le_antisymm (Nat.find_mono fun n => Exists.imp fun l => And.imp_right le_of_eq) <|
+    (Nat.le_find_iff _ _).2 fun k hk ⟨l, hl, hlk⟩ => (Nat.lt_find_iff _ _).1 hk _ hlk ⟨l, hl, rfl⟩
+
+@[to_additive]
+noncomputable instance : NormedGroup (MarkedGroup m) where
+  norm x := ‖quotientEquivMarkedGroup.symm x‖
+
+namespace MarkedGroup
+
+@[to_additive add_norm_def]
+private lemma norm_def [DecidableEq S] (x : MarkedGroup m)
+    [DecidablePred fun n ↦ ∃ l, toMarkedGroup (m l) = x ∧ l.toWord.length = n] :
+    ‖x‖ = Nat.find (mul_aux' x) := by
+  convert congr_arg Nat.cast (find_mul_aux _)
+  classical
+  infer_instance
+
+@[to_additive add_norm_def]
+lemma norm_le_iff [DecidableEq S] (x : MarkedGroup m)
+    [DecidablePred fun n ↦ ∃ l, toMarkedGroup (m l) = x ∧ l.toWord.length = n] :
+    ‖x‖ = Nat.find (mul_aux' x) := by
+  convert congr_arg Nat.cast (find_mul_aux _)
+  classical
+  infer_instance
+
+end MarkedGroup
diff --git a/Mathlib/Topology/Algebra/ConstMulAction.lean b/Mathlib/Topology/Algebra/ConstMulAction.lean
index d832fb597a5e12..91535addb0dd7e 100644
--- a/Mathlib/Topology/Algebra/ConstMulAction.lean
+++ b/Mathlib/Topology/Algebra/ConstMulAction.lean
@@ -202,6 +202,10 @@ theorem tendsto_const_smul_iff {f : β → α} {l : Filter β} {a : α} (c : G)
     Tendsto (fun x => c • f x) l (𝓝 <| c • a) ↔ Tendsto f l (𝓝 a) :=
   ⟨fun h => by simpa only [inv_smul_smul] using h.const_smul c⁻¹, fun h => h.const_smul _⟩
 
+@[to_additive]
+instance Subgroup.continuousConstSMul {S : Subgroup G} : ContinuousConstSMul S α :=
+  IsInducing.id.continuousConstSMul Subtype.val rfl
+
 variable [TopologicalSpace β] {f : β → α} {b : β} {s : Set β}
 
 @[to_additive]
diff --git a/Mathlib/Topology/Algebra/Group/Quotient.lean b/Mathlib/Topology/Algebra/Group/Quotient.lean
index 926b9d5d90bb29..8acce598eca5d9 100644
--- a/Mathlib/Topology/Algebra/Group/Quotient.lean
+++ b/Mathlib/Topology/Algebra/Group/Quotient.lean
@@ -44,9 +44,8 @@ theorem isQuotientMap_mk (N : Subgroup G) : IsQuotientMap (mk : G → G ⧸ N) :
 theorem continuous_mk {N : Subgroup G} : Continuous (mk : G → G ⧸ N) :=
   continuous_quot_mk
 
-section ContinuousMul
-
-variable [ContinuousMul G] {N : Subgroup G}
+section ContinuousMulConst
+variable [ContinuousConstSMul Gᵐᵒᵖ G] {N : Subgroup G}
 
 @[to_additive]
 theorem isOpenMap_coe : IsOpenMap ((↑) : G → G ⧸ N) := isOpenMap_quotient_mk'_mul
@@ -118,6 +117,21 @@ instance instLocallyCompactSpace [LocallyCompactSpace G] (N : Subgroup G) :
     LocallyCompactSpace (G ⧸ N) :=
   QuotientGroup.isOpenQuotientMap_mk.locallyCompactSpace
 
+end ContinuousMulConst
+
+section ContinuousMul
+
+variable [ContinuousConstSMul Gᵐᵒᵖ G] {N : Subgroup G}
+
+-- @[to_additive]
+-- instance instContinuousSMul : ContinuousSMul G (G ⧸ N) where
+--   continuous_smul := by
+--     rw [← (IsOpenQuotientMap.id.prodMap isOpenQuotientMap_mk).continuous_comp_iff]
+--     exact continuous_mk.comp continuous_mul
+
+-- @[to_additive]
+-- instance instContinuousConstSMul : ContinuousConstSMul G (G ⧸ N) := inferInstance
+
 variable (N)
 
 /-- Neighborhoods in the quotient are precisely the map of neighborhoods in the prequotient. -/