And what is a "deliberative assembly"? Robert defines a deliberative assembly as "a group of people, having or assuming freedom to act in concert, meeting to determine, in full and free discussion, courses of action to be taken in the name of the entire group." RONR, p.1 (11th ed.). Although a true deliberative assembly is a group generally larger than about a dozen people, smaller groups can meet and apply many of the same rules in a less formal setting.
Parliamentary procedure should be distinguished from the primary competing method of consensus decision-making in that the latter does not depend on a majority vote of the participants. There is certainly nothing wrong with making decisions based on a group's consensus (without defining precisely what this means) but consensus-driven models of decision-making are usually not possible in large groups with very vocal factions or significant minority positions. In those situations, Parliamentary Procedure has shown itself to be the time-honored method for group decision-making that does not depend primarily on reaching an "overall consensus" besides a majority (and sometimes supermajority) vote.
For those unfamiliar with Parliamentary Procedure, the process may at first-glance seem complex and technical. However, a good parliamentarian (someone who is trained in parliamentary procedure and has experience facilitating meetings using this method) can successfully guide a meeting with a minimal amount of instruction, even where the participants are relatively ignorant of the details. There are also books such as Robert's Rules of Order in Brief that are inexpensive, quick to read, and can be used by the membership to master enough of the essentials to make the meeting an overall success.
There is not one single method of parliamentary law, with the many different methods comprising what has been called the "common parliamentary law." Links on this website have been provided for some of the more important historical treatises on the subject. However, because of the differences that may exist between any particular code of parliamentary procedure, it is advisable to pick one particular treatise and stick with it as the official parliamentary authority for the group; the other treatises can act in an advisory capacity as persuasive authority should the treatise that is adopted as the mandatory authority be unclear on any particular point. Of course, on this website we recommend the Eleventh Edition of Robert's Rules of Order Newly Revised as the most complete compendium of contemporary parliamentary procedure for adoption as the legal authority on the matter for inclusion in any organization's bylaws.