A centrifuge rotor is the part of the centrifuge that holds the sample tube during processing. The rotor is designed for a specific tube size, machine size, and sometimes brand and model. For example, a microcentrifuge designed for use with microtubes will use a different rotor than will a floor-standing model designed to process larger sample sizes. Swing-bucket rotors allow operators to place samples while the rotor buckets are a vertical position, easing and speeding loading and unloading. They are generally used for large-volume samples requiring higher speeds and in applications processing high quantities of samples. Fixed-angle rotors are generally for processing conical-shaped tubes and applications that require pellet-formation of the extracted material. Centrifuge rotors are chosen based on the type of substance being processed, sample tube size, and the bucket type and capacity.