At the present day by far the largest proportion of sheet rubber is prepared in machines known as calenders. Indeed, the calenders are among the most important machines in modern rubber factories. A calender consists of a series of heavy rollers revolving alternately in opposite directions. A common type has three rollers arranged one above the other. The rubber mixture is fed between the upper and middle rollers. The sheet thus formed, varying in thickness according to the distance at which the rollers are set. adheres to the middle roller and passes back between the middle and lower rollers. For packing the sheet thus obtained, the following method is adopted. A sheet of cloth is unwound from a drum on one side of the machine. The cloth passes between the middle and lower rollers, and is rolled up together with the rubber sheet on the opposite side of the machine. If it is desired to make the rubber adhere closely to the cloth, the middle and lower rollers are made to revolve at different rates of speed, and the friction thus caused produces the desired adhesion. The rubber is then said to be "frictioned" on to the cloth. Frictioned sheet is used in the manufacture of a large variety of articles in which alternate layers of cloth and rubber have to be incorporated.