Information transferred between processes is referred to as "mail."
It is sent from one process to another through an intermediate
storage area called a "mailbox." At any given time, a mailbox can
contain only one item of mail (a "message"). For any process,
there are two sets of mailboxes:
The mailbox used for communication between the process and its parent; each process has one of these.
The set of mailboxes used for communication between the process and its children; each process has one of these mailboxes for each of its children.
Even though there are two sets of mailboxes, there is only one
mailbox between any two processes.
