Joint activities are essentially a complex of coordination problems [13]. Playing a duet, shaking hands, rowing a boat in tandem, eating dinner at a restaurant, conversing with another, and so on, are all joint activities that require that the participants solve a set of coordination problems.
Joint activities have participants, who assume public roles. There are joint public goals and private ones. Joint activities emerge as a hierarchy of joint actions and joint (sub)activities. The entry and exit boundaries of a joint activity are jointly engineered by the participants. Joint activities advance one increment at a time, mostly through joint actions. Joint actions are created when people coordinate with each other. Joint actions have phases that have entry and exit points, each of which require coordination. Coordination on the entry and exit times to each phase can be achieved by means of different strategies.
An example of a joint activity is Sam and Gladys, two teenagers on a date engineering a `first kiss'. Sam and Gladys are the participants. Sam's public role is `Romeo' and Gladys' public role is `Juliet'. Their joint public goal is to kiss; their private goals may differ. Entry into the first stage of kissing is jointly engineered by the participants. Sam and Gladys are doing their parts in the joint action of initiating a kiss. Coordination of entry and exit times for each of the phases, from the initial touch, to looking into each others eyes, to the first touch of lips, is jointly achieved. The joint action (activity) of Sam and Gladys initiating a first kiss is a complex of coordination problems.
What accumulates during a joint activity is common ground. At any moment during a joint activity, what constitutes common ground has three parts: initial common ground, current state of the joint activity, and public events so far. The initial common ground is the set of background facts, assumptions, and beliefs presupposed at the outset of the joint of activity. The current state of the joint activity is where in the activity the participants presuppose themselves to be. The public events are those events presupposed by the participants as leading up to the current state of affairs.
The initial common ground for Sam and Gladys' kissing includes the cultural history within their community for the activity of `first kissing' and the prior dating behaviors of their clique. As Sam and Gladys proceed through the activity, changes in state slowly, inexorably, move from assumptions and beliefs that the `first kiss' might happen to the confirmation that it has. Events like Sam putting his hand on Gladys' shoulder, and Gladys reciprocating by putting her hand on his hip, are `public' events which mark the progress of the couple in their joint activity.