The M109 is an American self-propelled, 155 mm turreted howitzer, first developed to replace the M44 in the early 1960s. It is upgraded several times, most recently to the M109A7. The M109 family is the most common Western indirect-fire support weapon in armored and mechanized divisions of infantry maneuvering brigades. The M109 has a crew of four: the chief / commander of the team, the pilot, the gunner, and the handler / loader of munitions. The chief or gunner targets the left or the right cannon (deflection) and up and down (quadrant)