How a Trombone is Made
Stockings on the tubing!?

Although slides are made to be as straight as possible, the ends of the inner slide are actually barrel-shaped.
The variation is subtle enough that you wouldn't notice it unless you were looking for it, but the last 10 cm or so from each end is slightly thicker. These are called the stockings. Note also that the middle sections of the inner slide are slightly thicker and barrel-shaped in Yamaha instruments.

Stockings are the parts on the ends of the inner slides that are slightly thicker (roughly 0.3 mm in diameter). They are modern inventions. Roughly 10 cm long, these parts of the inner slide rub and slide against the outer slide.
Trombones long ago were not like this, and the entire length of the inner slide rubbed against the outer slide. It is likely that this is because it was difficult at the time to shape the inner slide so that it was slightly thicker on the ends-or possibly no one thought to do this back then. Trombones back then, therefore, were hard to slide and difficult to play melodies on. Stockings have drastically changed the way trombones are played.
The stockings are barrel shaped to reduce the portion that is actually rubbed. If the tubing were completely straight its entire surface would make contact, and there would be more resistance due to friction. The barrel shape is designed to make only line contact.