machine used to separate the fibers of cotton from the seeds. The American inventor Eli Whitney is generally credited with inventing the cotton gin in 1793. Before the invention of the cotton gin, seeds had to be removed from cotton fibers by hand; this labor-intensive and time-consuming process made growing and harvesting cotton uneconomical. The cotton gin allowed the seeds to be removed mechanically and rapidly from the cotton fibers, making cotton production economical and leading to dramatic growth in the United States cotton industry. This expansion contributed to an increase of slave labor in the United States.

Whitney's cotton gin, also called a saw gin, consisted of a cylinder to which a number of saw like teeth were attached. As the cylinder revolved, the teeth passed through the closely spaced ribs of a fixed comb. When cotton was fed into the gin, the teeth caught the cotton fibers and pulled them through the comb. The seeds, which were too large to pass between the ribs, were left behind. This principle, with virtually no modifications, is still employed in modern automatic saw gins used to process the bulk of the U.S. cotton crop.

One disadvantage of the saw gin is that it tends to damage the fiber, particularly in the case of long-staple cottons. For ginning such cottons, which include the Egyptian, pima, and Sea Island varieties, the roller gin is used. In the roller gin the cotton is carried on the surface of a leather-covered roller that has a blade fixed parallel to the axis of the roller and nearly touching its surface. The cotton fiber passes under the blade on the roller, but the seeds cannot pass the blade and are forced out of the fiber. The roller gin is slow, so it is used only for premium grades of cotton.