A brigade, on the other hand, is a flexible organization; it has no permanent elements. A brigade may have several different kinds of units assigned to it, such as: three light infantry battalions or two mechanized infantry and an armor battalion or one light, one mechanized and one armor battalion; plus support units. The usual number of maneuver battalions was three; however, this was a guideline not a rule (ex: the 173rd Airborne Brigade in Vietnam had four airborne infantry battalions).