In normal interactive usage, the input and output sides of the bidirectional stream *terminal-io* are hooked up to the the operating system's standard input and standard output. The lisp streams *standard-input*, *standard-output*, and *error-output* are synonym streams for *terminal-io*.

In batch mode, this arrangement is modified slightly. The lisp streams *standard-input*, *standard-output*, and *standard-error* correspond directly to the operating system's standard input, standard output, and standard error. If the lisp can determine that it has access to an operating system tty, then *terminal-io* will be hooked up to that. Otherwise, the input and output streams of *terminal-io* will correspond to the operating system's standard input and standard output.