The types of rolling mills are often classified based on the number of rollers within the mill.
Two-high rolling mills consist of two horizontal rolls placed on top of each other. Either one or both of the rollers are adjustable. Some two-high rolling mills have the rollers rotating in the same direction, while others have them spinning in different directions.
Three-high rolling mills consist of three horizontal roles placed in a single vertical plane. The upper and bottom roller roll in the same direction while the middle one rotates in the opposite direction.
The four-high rolling mill consists of four rollers: two working rollers of smaller diameter and two back up rollers with a larger diameter. All four rollers are arranged above one another in a vertical plane.
Cluster rolling mills are a specific type of four-high rolling mill in which each of the working rolls is backed up by two or more of the larger backup rolls (instead of a single backup roll in the typical four-high rolling mill).