diff --git a/CHANGES.md b/CHANGES.md
index f1bd828..f6e6bdc 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,7 @@
+### Unreleased
+
+- Add `Pidign.Pris` for bi-directional definition (by [mspwn](https://github.com/mspwn) and [xvw](https://github.com/xvw))
+
 ### v1.0.0
 
 #### Pidgin
diff --git a/lib/pidgin.ml b/lib/pidgin.ml
index 27b5f0b..bd21cd0 100644
--- a/lib/pidgin.ml
+++ b/lib/pidgin.ml
@@ -6,6 +6,7 @@
 module Repr = Repr
 module Kind = Kind
 module Check = Check
+module Prism = Prism
 module Driver = Driver
 module Sexp = Sexp
 module Csexp = Csexp
diff --git a/lib/pidgin.mli b/lib/pidgin.mli
index 3912fa8..e51c75d 100644
--- a/lib/pidgin.mli
+++ b/lib/pidgin.mli
@@ -11,9 +11,10 @@
     to that of JSON) and to provide:
 
     - a DSL for describing arbitrary data structures in this language
-      (see {!module:Repr})
+      (see {!module:Repr}).
     - Validation functions that operate on data described using this
-      DSL (see {!module:Check})
+      DSL (see {!module:Check}).
+    - [Repr] and [Check] can be combined using {!module:Prism}.
     - A bidirectional conversion approach (see {!module:Driver}),
       imposing a cost due to the indirect nature of the generic format
       (though it is viable in many scenarios).
@@ -48,4 +49,5 @@ module Csexp = Csexp
 
 (** {1 Misc} *)
 
+module Prism = Prism
 module Misc = Misc
diff --git a/lib/prism.ml b/lib/prism.ml
new file mode 100644
index 0000000..300e6ea
--- /dev/null
+++ b/lib/prism.ml
@@ -0,0 +1,25 @@
+(* Copyright (c) 2026, Cargocut and the Pidgin developers.
+   All rights reserved.
+
+   SPDX-License-Identifier: BSD-3-Clause *)
+
+type 'a t =
+  { conv : 'a Repr.conv
+  ; check : 'a Check.t
+  }
+
+let make ~conv ~check = { conv; check }
+let conv { conv; _ } = conv
+let check { check; _ } = check
+
+let invmap f g { conv; check } =
+  { check = (fun x -> x |> check |> Result.map f)
+  ; conv = (fun x -> x |> g |> conv)
+  }
+;;
+
+let refine f g { conv; check } =
+  { check = (fun x -> Result.bind (check x) f)
+  ; conv = (fun x -> x |> g |> conv)
+  }
+;;
diff --git a/lib/prism.mli b/lib/prism.mli
new file mode 100644
index 0000000..926cc51
--- /dev/null
+++ b/lib/prism.mli
@@ -0,0 +1,38 @@
+(* Copyright (c) 2026, Cargocut and the Pidgin developers.
+   All rights reserved.
+
+   SPDX-License-Identifier: BSD-3-Clause *)
+
+(** A [Prism] is used to describe a codec that enables serialization
+    and deserialization (it consists of a {!type:Repr.conv} and a
+    {!type:Check.t} pair). *)
+
+(** {1 Types} *)
+
+(** The type that describes the pair of converter and validator. *)
+type 'a t
+
+(** {1 Building prisms} *)
+
+(** [make ~conv ~check] constructs a prism for type ['a]. *)
+val make : conv:'a Repr.conv -> check:'a Check.t -> 'a t
+
+(** {1 Running prisms} *)
+
+(** [conv prism] extract the prism conversion function. *)
+val conv : 'a t -> 'a Repr.conv
+
+(** [check prism] extract the prism validation function. *)
+val check : 'a t -> 'a Check.t
+
+(** {1 Helpers}
+
+    A few additional tools for working with prisms. *)
+
+(** [invmap f g prism] map both [f] and [g] on, respectively, [check]
+    and [conv]. *)
+val invmap : ('a -> 'b) -> ('b -> 'a) -> 'a t -> 'b t
+
+(** [refine f g prism] bind [f] and map [g] on, respectively, [check]
+    and [conv]. *)
+val refine : ('a, 'b) Check.fn -> ('b -> 'a) -> 'a t -> 'b t