We define Human-computer interaction (HCI) etiquette rules as a type of software design guideline aimed at facilitating smooth and effective interactions between humans and computers. They do so, by insuring that the software follows expected rules of interaction appropriate to the computer’s role in a team. In particular, HCI etiquette rules from the cultural conventions of a particular group. In design of decision support tools to assist specific work groups such as a military intelligence analysts, a team of product designers, or the crew of a helicopter, it is particularly important to understand the sub-culture of that work group. In many domains, following the "etiquette" of the work groups can be key in getting human users to accept the software as part of the team, and to use its benefits. Software that does not follow rules of HCI etiquette, even if otherwise well designed may be rejected by users. For example, if the software interrupts too much during crisis periods, or fails to provide the type of interactions, such as justifications, or redirection of goals, that the user typically expects of his or her subordinates. The term human computer etiquette was first introduced by Miller and Funk. In this paper, our goals are to refine their definition of HCI etiquette, and define it as one (of many) types of HCI design rules, and to establish the necessity benefits of viewing HCI etiquette as an explicit category.