Current American combat divisions have 25-30 battalions assigned, with half of them being combat units. These battalions are organized into seven brigade size organizations, plus the division headquarters (which often controls the Military Intelligence battalion, Signal Battalion and several company size units.) Over the last decade, American army combat divisions have undergone some subtle changes in organization and operation. One of the more noticeable differences has been the appearance of the "fourth brigade." Since the 1960s, Army divisions referred to their "three brigades" to mean the three brigades that contained their infantry and armor battalions. But in the 1980s, the aviation battalion the divisions had since the 1960s, was expanded to an aviation brigade. In the 1990s, it became customary to assign the division reconnaissance battalion to the aviation brigade as well. This meant that a brigade that usually had a transport helicopter battalion and an attack helicopter battalion now had a ground unit (the cavalry squadron, which was a battalion size unit). In some divisions, the aviation brigade sometimes had a tank or infantry battalion assigned as well, at least for training exercises or combat operations. During the 1990s, tank and mechanized divisions were given two more engineer battalions, with the three engineer battalions now organized into an engineer brigade. Although these are combat engineers, the three battalions are usually assigned to one of the four combat brigades. There are two other "support brigades"; the divisional artillery (with four artillery battalions) and the divisional support command (with six or more support and maintenance battalions.) Manpower in US divisions now runs close to 20,000 troops with the attachment of additional artillery and aviation battalions. Sometimes yet another combat brigade is attached as well.