What is the role of a shared whiteboard in building a shared understanding of the task and its solution? Our hypothesis was that a graphical communication tool would facilitate grounding processes, i.e. the mutual understanding of one or a few utterances and thereby the construction of a shared solution. We conducted an empirical study with 20 pairs solving an enigma in a MUD environment enriched with a whiteboard. The results show that the whiteboard was used less used to disambiguate difficult concepts in conversation than as a tool for distributed regulation of the task. The graphical features of the whiteboard were less exploited thatn the fact that information displayed on the whiteboard was persistent. Subjects selected the communication medium by matching the persistency of display (how long information is displayed) with persistency of information (how long it remains valid). Their grounding behaviour also takes into account the probability that some piece of information probabilty that some piece of infornmation is misundersis misunderstood or disagreed upon.