The concept of a Message class is briefly discussed in the Blue Book, but no details are provided about its protocol. CLDT defines both a Message class and a DirectedMessage class. Both are used in error handling and to support communication between the virtual machine and the rest of the system. A Message represents a Smalltalk message. It supports the basic accessors arguments, arguments:, selector, and selector:. A DirectedMessage is a Message that also knows about its receiver. It supports the basic accessors plus receiver, receiver:, and send.