diff --git a/networks.rkt b/networks.rkt
index 8f1bf78..441e2d5 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -8,21 +8,22 @@
     (require typed/rackunit))
 
   (provide
-   State UpdateFunction DomainMapping
+   State UpdateFunction Domain DomainMapping
    (struct-out network) Network
 
    make-same-domains make-boolean-domains make-boolean-network)
 
   (define-type (State a) (VariableMapping a))
   (define-type (UpdateFunction a) (-> (State a) a))
-  (define-type (DomainMapping a) (VariableMapping (Listof a)))
+  (define-type (Domain a) (Listof a))
+  (define-type (DomainMapping a) (VariableMapping (Domain a)))
 
   (struct (a) network ([functions : (VariableMapping (UpdateFunction a))]
                        [domains : (DomainMapping a)])
     #:transparent
     #:type-name Network)
 
-  (: make-same-domains (All (a) (-> (Listof Variable) (Listof a)
+  (: make-same-domains (All (a) (-> (Listof Variable) (Domain a)
                                     (DomainMapping a))))
   (define (make-same-domains vars domain)
     (for/hash ([var vars]) : (DomainMapping a)
diff --git a/scribblings/networks.scrbl b/scribblings/networks.scrbl
index 74d6656..582e7ef 100644
--- a/scribblings/networks.scrbl
+++ b/scribblings/networks.scrbl
@@ -41,6 +41,14 @@ state.  This is a synonym of the type @racket[(-> (State a) a)].
 
 }
 
+@defform[#:kind "type" (Domain a)]{
+
+A domain which is a subset of the type @racket[a].
+
+@racket[(Domain a)] is a synonym of @racket[(Listof a)].
+
+}
+
 @defform[#:kind "type" (DomainMapping a)]{
 
 A domain mapping is a hash table mapping variables to the lists of values in
@@ -80,7 +88,7 @@ The type of the instances of @racket[Network].
 @section{Constructing networks}
 
 @defproc[(make-same-domains [vars (Listof Variable)]
-			    [domain (Listof a)])
+			    [domain (Domain a)])
 			    (DomainMapping a)]{
 
 Makes a hash set mapping all variables to a single domain.