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coordination of content. They must also synchronize their entrances and
exits, coordinate how loud to play forte and pianissimo, and otherwise
adjust to each other's tempo and dynamics. This is coordination of pro-
cess. They cannot even begin to coordinate on content without assuming
a vast amount of shared information or common ground—that is, mu-
tual knowledge, mutual beliefs, and mutual assumptions (see Clark and
Carlson 1982, Clark and Marshall 1981, Lewis 1969, Schelling 1960). And
to coordinate on process, they need to update, or revise, their common
ground moment by moment. All collective actions are built on common
ground and its accumulation.
In her work in applying the notion of common ground to human-
computer interfaces, Brennan (1990a) suggests that common ground is a
jointly inhabited “space” in which meaning takes shape through the col-
laboration and successive approximations of the participants. Brennan's
work was aimed at designing human-computer interfaces so that they offer
means for establishing common ground (“grounding”) that are similar to
those that people use in human-to-human conversation, such as interrup-
tions, questions, and utterances and gestures that indicate whether some-
thing is being understood (Brennan 1990b).
Successful graphical interfaces, exemplifi ed early on by the Macintosh,
explicitly represented part of what Clark called the “perceptual common
ground” of interaction through the appearance and behavior of objects
on the screen (Clark 1996). Some of what goes on in the representation is
exclusively attributable to either the person or the computer, and some of
what happens is a virtuous artifact of a collaboration in which the traits,
goals, and behaviors of both are inseparably intertwined.
The concept of common ground not only provides a superior model of
the conversational process, but it also supports the idea that an interface is
not simply the means whereby a person and a computer represent them-
selves to one another; rather, it forms a shared context for action in which
both are agents.
1
When the old tit-for-tat paradigm intrudes, the “conversa-
tion” is likely to break down, once again relegating person and computer
to opposite sides of a “mystic gulf”
2
fi lled with hidden processes, arbitrary
1. This topic employs the noun “agent” to mean
one who initiates action
. This defi nition is con-
sistent with Aristotle's use of the concept in the
Poetics
.
2. The term “mystic gulf” is attributed to composer Richard Wagner to refer to the gap between
audience and actors created by the orchestra pit.
