The Court's first explicit statement of the designated public forum doctrine came in Southeastern Promotions v Conrad.  Southeastern Promotions sought permission to use Chattanooga, Tennessee's municipal auditorium for performances of the musical "Hair."  Although the auditorium had been rented for a wide variety of expressive activities prior to Southeastern's application, Chattanooga city officials refused Southeastern's request, citing Hair's nudity, tacit approval for drug use, sexual themes, and bad language.  The Court found the municipal auditorium to be a designated public forum, and the city's refusal to permit use of its auditorium to be an unconstitutional prior restraint.

The main difference between  traditional and designated public forum, for First Amendment purposes, is that the government, in dedicating the forum for expressive purposes--that is, in defining the forum--may adopt reasonable limitations on who may use the forum. (Also, government cannot ban expression completely in the traditional public forum, whereas it is under no First Amendment obligation to have opened a designated forum.) For example, the UMKC student center meeting rooms found to be a designated public forum in Widmar v Vincent could be restricted in their use to students, but the university could not--the Court said--allow students to meet for academic, social, or political purposes, but not for religious purposes.

Rosenberger v University of Virginia demonstrates that a designated public forum need not be a physical place.  In Rosenberger, the Court found that Virginia had created a designated public forum when it established a fund that would cover the cost of publications by eligible student groups.  Once having created such a forum (which, of course, it was under no obligation to do), Virginia could not refuse funding to a student organization because of the overtly religious nature of its publication.