The windage and elevation adjustment knobs (colloquially called "turrets") often have internal ball detents to help accurately index their rotation, which provide a crisp tactile feedback corresponding to each graduation of turn, often accompanied by a soft but audible clicking sound. Each indexing increment is thus colloquially called a "click", and the corresponding angular adjustment of the optical axis is known as the "click value". The most commonly seen click values are  MOA (often expressed in approximations as " inch at 100 yards") and 0.1 mil (often expressed as "10 mm at 100 meters"), although other click values such as  MOA,  MOA or  MOA and other mil increments can also be infrequently seen on the commercial and military and law enforcement markets. A common problem with the elevation and windage adjustments is that formerly smooth-working turrets can "get stuck" over the years, and is generally caused by long-term lack of movement in the lubricated turret mechanisms, which caused the ball detents to gall and stagnate.