Editor-In-Chief: C. Michael Gibson, M.S., M.D. ;Associate Editor(s)-in-Chief: Kiran Singh, M.D. 
A callus (or callous) is an especially toughened area of skin which has become relatively thick and hard as a response to repeated contact or pressure. In botany, the term is also used to announce a condition of thickened surfaces of leaves or other plant parts. Since repeated contact is required, calluses are most often found on hands or feet. Calluses are generally not harmful, but may sometimes lead to other problems, such as infection. Shoes that fit tightly can often produce calluses on the feet. String instrument players develop calluses where their fingers make contact with the strings, but these calluses are often desirable as they help alleviate the pain from the tension of the strings and make playing easier. Dancers often develop calluses on the soles of their feet from dancing barefoot, which makes performing turns less difficult. Video game players may develop calluses on their thumbs, especially if using a controller with a poor D-pad. Frequent use of a writing implement may lead to a callus on the middle finger, commonly known as a "writer's bump".
Corns (also called clavi) are specially-shaped calluses of dead skin that usually occur on thin or glabrous (hairless and smooth) skin surfaces, especially on the dorsa of toes or fingers. They can sometimes occur on the thicker palmar or plantar skin surfaces. Corns form when the pressure point against the skin traces an elliptical or semi-elliptical path, the center of which is at the point of pressure, gradually widening. If there is constant stimulation of the tissues producing the corns, even after the corn is removed or the pressure surgically removed, the skin may continue to grow as a corn.
The name corn comes from its appearance under the microscope. The hard part at the center of the corn resembles a barley hare, that is, a funnel with a broad raised top and a pointed bottom. "Corn" used to be a generic term for grain, and the name stuck. The scientific name is heloma. Hard corns are called heloma durum, while soft corns are called heloma molle.
The place of occurrence differentiates between soft and hard corns. Hard corns occur on dry, flat surfaces of skin. Soft corns (frequently found between adjacent toes) stay moist, keeping the surrounding skin soft. The corn's center is not soft, however, but indurated.
Although usually found on the foot (where the most pressure occurs), calluses can occur anywhere on the body as a reaction to moderate, constant "grinding" pressure. It is the natural reaction of the palmar or plantar skin.
For example, players of string instruments will develop calluses on their fingers with frequent play. This actually helps the player as the thicker skin protects the fingertips - extended play is often painful before the calluses appear. If a beginning player plays too hard, or with an extended amount of left- (especially) or right-hand pizzicato, however, a blister may be produced instead. Drummers can also develop a callus on their feet and hands. Calluses on the feet are more common in metal drummers, where double bass drumming is used more often. Use of older sticks will also cause callus on the palms and fingers of a drummer.
People with bunions may find painful calluses behind the second or third toe. These are caused by unequal pressure placed on the smaller toes. Such pressure-induced calluses can be very painful and often do not respond to trimming of the callus, soft materials, or orthotic devices. It is not the callus that causes pain, but rather the severe imbalance in the function of the foot that is taking its toll.
Shoes can produce corns by rubbing against the top of the toes or foot. Continued irritation may cause pain. Stretching the rubbing area of the shoe may reduce the pressure and stop the pain, but the corn may remain. If a toenail or a fingernail rubs against the skin, pinching it between surfaces for a period of time, a corn can form at the edge of the nail. These are difficult to treat because frequently the nail is the primary cause.
Sometimes a callus occurs where there is no rubbing or pressure. These hyperkeratoses can have a variety of causes. Some toxins, such as arsenic, can cause thick palms and soles. Some diseases, such as syphilis, can cause thickening of the palms and soles as well as pinpoint hyperkeratoses. There is a benign condition called keratosis palmaris et plantaris, which produces corns in the creases of the fingers and non-weight-bearing spaces of the feet. Some of this may be caused by actinic keratosis, which occurs due to overexposure to sun, or with age and hormonal shifts.
A common method, often done by a podiatrist, is to shave the calluses down, and perhaps pad them.
For calluses on the feet an inexpensive home remedy is to dissolve a foot soap powder composed of borax, iodine and bran in warm water and soak the feet in the solution for 15 to 20 minutes. This softens the calluses so that layers of dead skin can be rubbed away with a cloth towel. Repeated soaking over a period of several days can often allow removal of even the core with nothing more than the friction of the cloth towel. If this fails, use of a pumice stone can also remove the skin.
Most corns and calluses located under the foot are caused by the pressure of the foot bones against the skin, preventing it from moving with the shoe or the ground. While well-fitting shoes will help some of these problems, occasionally some other degree of intervention is required to completely rid the foot of the problem. The most basic treatment is to put a friction-reducing insole or material into the shoe, or against the foot. In some cases, this will reduce the painfulness without actually making the callus go away.
In many situations, a change in the function of the foot by use of an orthotic device is required. This reduces friction and pressure, allowing the skin to rest and to stop forming protective skin coverings.
Salicylic acid (0.5%-40%) can be used for two reasons, "(1) it decreases keratinocyte adhesion, and (2) it increases water binding which leads to hydration of the keratin." 
Using a knife to cut it away is dangerous because it can result in bleeding of the foot and infection.
At other times, surgical correction of the pressure is needed.
People with diabetes face special skin challenges. Because diabetes affects the capillaries, the small vessels which feed the skin its blood supply, thickening of the skin increases the difficulty to supply nutrients to the skin. Additionally, the shear and pressure forces that cause corns and calluses may tear the capillaries, causing bleeding within the callus or corn.
Often, bleeding within the calluses is an early sign of diabetes, even before elevated blood sugars. Although the bleeding can be small, sometimes small pools of blood or hematoma are formed. The blood itself is an irritant, a foreign body within the callus that makes the area burn or itch. If the pool of blood is exposed to the outside, infection may follow. Infection may lead to ulceration. Luckily, this process can be prevented at several places, but such infections can become life-threatening. Diabetic foot infections are the leading cause of diabetic limb amputation.
de:Hornschwiele it:Callo nl:Eelt fi:Liikavarvas