This paper describes the architecture of a proposed document composition system named JANUS, which is intended to provide support for authors of complex documents containing mixtures of text, line art, and tone art. The JANUS system is highly interactive, providing authors with immediate feedback and direct electronic control over page layouts, using a special two-display workstation. Authors communicate with the system by marking up their documents with high-level descriptive “tags”. A tag definition language is provided whereby new tags may be defined and the format of each tagged object may be controlled. © 1981, ACM. All rights reserved.