diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0e9cd648ab..5091c14691 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3042,3 +3042,15 @@ This is a full list of proofs that have changed form to use irrelevant instance
   ```agda
   ↭-reverse : (xs : List A) → reverse xs ↭ xs
   ```
+
+* Added new file `Relation.Binary.Reasoning.Base.Apartness`
+
+This is how to use it:
+  ```agda
+  _ : a # d
+  _ = begin-apartness
+    a ≈⟨ a≈b ⟩
+    b #⟨ b#c ⟩
+    c ≈⟨ c≈d ⟩
+    d ∎
+  ```
diff --git a/src/Relation/Binary/Reasoning/Base/Apartness.agda b/src/Relation/Binary/Reasoning/Base/Apartness.agda
new file mode 100644
index 0000000000..7a52568664
--- /dev/null
+++ b/src/Relation/Binary/Reasoning/Base/Apartness.agda
@@ -0,0 +1,106 @@
+------------------------------------------------------------------------
+-- The Agda standard library
+--
+-- The basic code for equational reasoning with three relations:
+-- equality and apartness
+------------------------------------------------------------------------
+
+{-# OPTIONS --cubical-compatible --safe #-}
+
+open import Relation.Binary
+
+module Relation.Binary.Reasoning.Base.Apartness {a ℓ₁ ℓ₂} {A : Set a}
+  {_≈_ : Rel A ℓ₁} {_#_ : Rel A ℓ₂}
+  (≈-equiv : IsEquivalence _≈_)
+  (#-trans : Transitive _#_) (#-sym   : Symmetric _#_)
+  (#-≈-trans : Trans _#_ _≈_ _#_) (≈-#-trans : Trans _≈_ _#_ _#_)
+  where
+
+open import Level using (Level; _⊔_)
+open import Relation.Binary.PropositionalEquality.Core
+  using (_≡_; refl; sym)
+open import Relation.Nullary.Decidable using (Dec; yes; no)
+open import Relation.Nullary.Decidable using (True; toWitness)
+
+open IsEquivalence ≈-equiv
+  renaming
+  ( refl  to ≈-refl
+  ; sym   to ≈-sym
+  ; trans to ≈-trans
+  )
+
+infix 4 _IsRelatedTo_
+
+data _IsRelatedTo_ (x y : A) : Set (a ⊔ ℓ₁ ⊔ ℓ₂) where
+  nothing   :                 x IsRelatedTo y
+  apartness : (x#y : x # y) → x IsRelatedTo y
+  equals    : (x≈y : x ≈ y) → x IsRelatedTo y
+
+data IsApartness {x y} : x IsRelatedTo y → Set (a ⊔ ℓ₁ ⊔ ℓ₂) where
+  isApartness : ∀ x#y → IsApartness (apartness x#y)
+
+IsApartness? : ∀ {x y} (x#y : x IsRelatedTo y) → Dec (IsApartness x#y)
+IsApartness? nothing         = no λ()
+IsApartness? (apartness x#y) = yes (isApartness x#y)
+IsApartness? (equals x≈y)    = no (λ ())
+
+extractApartness : ∀ {x y} {x#y : x IsRelatedTo y} → IsApartness x#y → x # y
+extractApartness (isApartness x#y) = x#y
+
+data IsEquality {x y} : x IsRelatedTo y → Set (a ⊔ ℓ₁ ⊔ ℓ₂) where
+  isEquality : ∀ x≈y → IsEquality (equals x≈y)
+
+IsEquality? : ∀ {x y} (x≲y : x IsRelatedTo y) → Dec (IsEquality x≲y)
+IsEquality? nothing       = no λ()
+IsEquality? (apartness _) = no λ()
+IsEquality? (equals  x≈y) = yes (isEquality x≈y)
+
+extractEquality : ∀ {x y} {x≲y : x IsRelatedTo y} → IsEquality x≲y → x ≈ y
+extractEquality (isEquality x≈y) = x≈y
+
+infix  1 begin-apartness_ begin-equality_
+infixr 2 step-≈ step-≈˘ step-≡ step-≡˘ step-# step-#˘ _≡⟨⟩_
+infix  3 _∎
+
+begin-apartness_ : ∀ {x y} (r : x IsRelatedTo y) → {s : True (IsApartness? r)} → x # y
+begin-apartness_ _ {s} = extractApartness (toWitness s)
+
+begin-equality_ : ∀ {x y} (r : x IsRelatedTo y) → {s : True (IsEquality? r)} → x ≈ y
+begin-equality_ _ {s} = extractEquality (toWitness s)
+
+step-# : ∀ (x : A) {y z} → y IsRelatedTo z → x # y → x IsRelatedTo z
+step-# x nothing  _          = nothing
+step-# x (apartness y#z) x#y = nothing
+step-# x (equals    y≈z) x#y = apartness (#-≈-trans x#y y≈z)
+
+step-#˘ : ∀ (x : A) {y z} → y IsRelatedTo z → y # x → x IsRelatedTo z
+step-#˘ x y-z y#x = step-# x y-z (#-sym y#x)
+
+step-≈  : ∀ (x : A) {y z} → y IsRelatedTo z → x ≈ y → x IsRelatedTo z
+step-≈ x nothing         x≈y = nothing
+step-≈ x (apartness y#z) x≈y = apartness (≈-#-trans x≈y y#z)
+step-≈ x (equals    y≈z) x≈y = equals    (≈-trans   x≈y y≈z)
+
+step-≈˘ : ∀ x {y z} → y IsRelatedTo z → y ≈ x → x IsRelatedTo z
+step-≈˘ x y∼z x≈y = step-≈ x y∼z (≈-sym x≈y)
+
+step-≡ : ∀ (x : A) {y z} → y IsRelatedTo z → x ≡ y → x IsRelatedTo z
+step-≡ x nothing _            = nothing
+step-≡ x (apartness x#y) refl = apartness x#y
+step-≡ x (equals    x≈y) refl = equals    x≈y
+
+step-≡˘ : ∀ x {y z} → y IsRelatedTo z → y ≡ x → x IsRelatedTo z
+step-≡˘ x y∼z x≡y = step-≡ x y∼z (sym x≡y)
+
+_≡⟨⟩_ : ∀ (x : A) {y} → x IsRelatedTo y → x IsRelatedTo y
+x ≡⟨⟩ x-y = x-y
+
+_∎ : ∀ x → x IsRelatedTo x
+x ∎ = equals ≈-refl
+
+syntax step-#  x y∼z x#y = x #⟨  x#y ⟩ y∼z
+syntax step-#˘ x y∼z x#y = x #˘⟨ x#y ⟩ y∼z
+syntax step-≈  x y∼z x≈y = x ≈⟨  x≈y ⟩ y∼z
+syntax step-≈˘ x y∼z y≈x = x ≈˘⟨ y≈x ⟩ y∼z
+syntax step-≡  x y∼z x≡y = x ≡⟨  x≡y ⟩ y∼z
+syntax step-≡˘ x y∼z y≡x = x ≡˘⟨ y≡x ⟩ y∼z