diff --git a/scribblings/tbn.scrbl b/scribblings/tbn.scrbl
index a6654ff..6d96ac1 100644
--- a/scribblings/tbn.scrbl
+++ b/scribblings/tbn.scrbl
@@ -277,6 +277,15 @@ Like @racket[tabulate-tbfs/state] and
 (tabulate-tbf/state+headers (tbf/state (hash 'a 1 'b 2) 2))
 ]}
 
+@section{TBNs and SBNs}
+
+@deftype[TBN]{
+
+The type of a TBN, i.e. a mapping assigning to each variable
+a @racket[TBF/State].
+
+}
+
 @section{Miscellaneous utilities}
 
 @defproc[(group-truth-table-by-nai [tt (Listof (Listof Integer))])
diff --git a/tbn.rkt b/tbn.rkt
index 286b69b..0ba0288 100644
--- a/tbn.rkt
+++ b/tbn.rkt
@@ -34,6 +34,8 @@
    tabulate-tbf/state tabulate-tbf/state+headers
 
    group-truth-table-by-nai
+
+   TBN
    )
 
   (: apply-tbf-to-state (-> TBF (State (U Zero One)) (U Zero One)))
@@ -317,6 +319,8 @@
                       ((0 0 1 1) (0 1 0 0) (1 0 0 0))
                       ((0 1 1 1) (1 0 1 0) (1 1 0 1))
                       ((1 1 1 0))))))
+
+  (define-type TBN (HashTable Variable TBF/State))
   )
 
 (module+ test