The ORD_KEY signature pairs an abstract key type with a comparison function that defines a total ordering on the keys. Ordered keys are used in the implementation of ordered maps (ORD_MAP) and ordered sets (ORD_SET).

signature ORD_KEY =
  sig
    type ord_key
        (* abstract type of keys that have a total order *)

    val compare : ord_key * ord_key -> order
        (* comparison function for the ordering on keys *)

  end (* ORD_KEY *)