diff --git a/networks.rkt b/networks.rkt
index e3508cc..b6dbd55 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -15,6 +15,8 @@
 (provide
  ;; Structures
  (struct-out dynamics)
+ (contract-out [struct state/tbf ([weights (hash/c variable? number?)]
+                                  [threshold number?])])
  ;; Functions
  (contract-out [update (-> network? state? (set/c variable? #:kind 'dont-care) state?)]
                [make-state (-> (listof (cons/c symbol? any/c)) state?)]
@@ -925,6 +927,10 @@
 ;;; TBF/TBN and SBF/SBN
 ;;; ===================
 
+;;; A state TBF is a TBF with named inputs.  A state TBF can be
+;;; applied to states in an unambiguous ways.
+(struct state/tbf (weights threshold))
+
 ;;; A TBN is a network form mapping TBFs to variables.
 (define tbn? (hash/c variable? tbf?))