Straightening machines today fall into two basic categories, the most common of which are known as "straighteners" or "flatteners". This arrangement is generally available in models with between 5 and 11 work rolls. The roll diameters and center distances vary depending on material thickness and width but straighteners and flatteners are generally distinguished by fairly large diameter, widely spaced rollers, usually not backed up. This kind of straightening machine is only capable of removing coil set from the material, thus allowing it to pass unrestricted through the die which satisfies most applications.

The second classification of straightening device is known as a "precision leveler". Precision levelers are distinguished by small diameter closely spaced rollers with backups and the ability to flex those rollers. They normally have a far greater number of work rollers than do conventional straighteners. Since they work the material much harder and their rollers can be flexed, precision levelers can be used to remove not only coil set but camber, wavy edges, center buckles, as well as trapped stresses within the material so that it will stay flat after processing through a die.