The type from which a subtype defines a subset of possible values, otherwise known as a constraint. This subset is not required to be proper. The base type of a type is the type itself. The base type of a subtype is found by recursively examining the type mark in the subtype indication defining the subtype. If the type mark denotes a type, that type is the base type of the subtype; otherwise, the type mark is a subtype, and this procedure is repeated on that subtype. See also subtype.