Design experience and theoretical discussion suggest that a narrow design focus on one tool or medium as primary may clash with the way that everyday activity involves the interweaving and combination of many heterogeneous media. Interaction may become seamless and unproblematic, even if the differences, boundaries and 'seams' in media are objectively perceivable. People accommodate and take advantage of seams and heterogeneity, in and through the process of interaction. We use an experiment with a mixed reality system to ground and detail our discussion of seamful design, which takes account of this process, and theory that reflects and informs such design. We critique the 'disappearance' mentioned by Weiser as a goal for ubicomp, and Dourish's 'embodied interaction' approach to HCI, suggesting that these design ideals may be unachievable or incomplete because they underemphasise the interdependence of 'invisible' non-rationalising interaction and focused rationalising interaction within ongoing activity
