cases infantry and the new armoured corps also operated their own integral light anti-aircraft artillery. Home defence anti-aircraft artillery often used fixed as well as mobile mountings. Some anti-aircraft guns could also be used as field or anti-tank artillery, providing they had suitable sights. However, the general switch by artillery to indirect fire before and during World War I led to a reaction in some armies. The result was accompanying or infantry guns. These were usually small, short range guns, that could be easily man-handled and used mostly for direct fire but some could use indirect fire. Some were operated by the artillery branch but under command of the supported unit. In World War II they were joined by self-propelled assault guns, although other armies adopted infantry or close support tanks in armoured branch units for the same purpose, subsequently tanks generally took on the accompanying role. Equipment types The three main types of artillery "gun" are guns, howitzers, and mortars. During the 20th century, guns and howitzers have steadily merged in artillery use, making a distinction between the terms somewhat meaningless. By the end of the 20th century, true guns with calibers larger than about 60 mm have become very rare in artillery use, the main users being tanks, ships, and a few residual anti-aircraft and coastal guns. The term "cannon" is a United States generic term that includes guns, howitzers, and mortars; it is not used in other English speaking armies. The traditional definitions differentiated between guns and howitzers in terms of maximum elevation (well less than 45° as opposed to close to or greater than 45°), number of charges (one or more than one charge), and having higher or lower muzzle velocity, sometimes indicated by barrel length. These three criteria give eight possible combinations, of which guns and howitzers are but two. However, modern "howitzers" have higher velocities and longer barrels than the equivalent "guns" of the first half of the 20th century. True guns are characterized by long range, having a maximum elevation significantly less than 45°, a high muzzle velocity and hence a relatively long barrel, smooth bore (no rifling) and a single charge. The latter often led to fixed ammunition where the projectile is locked to the cartridge case. There is no generally accepted minimum muzzle velocity or barrel length associated with a gun. Howitzers can fire at maximum elevations at least close to