Lighter and smaller than a conventional block, low friction rings or ferrules, have become a popular alternative for many applications where traditional blocks once ruled. Instead of rolling on a pulley sheave, the active line with a ring merely slides on the inside smooth surface of the ring.
Another line can be spliced around the ferrule for attaching it. This attachment line is sometimes made as a loop that is "luggage tagged" onto a pad-eye or the toe rail. Frictional losses are not as great as one may think, especially when using low friction line such as Dyneema.
In addition to being lighter and less expensive than conventional blocks, ring ferrules are also less prone to failure since there are no pulleys to crack or bearings to foul.