Derailers are much more interesting to look at and talk about, but it is still the chain that has to do the shifting. An old-style bicycle chain has ten parts per link. The typical 57 link chain used on the average multispeed bike has 570 parts, more than the whole rest of the bicycle put together. The major revolution in chain design has been the introduction of the bushingless chain. The first of this type was the Sedisport, and it has acquired such a good reputation that other manufacturers have copied the design.
Bushingless chains have only eight parts per link.
You cannot tell by looking at a chain on a bike if it is of bushingless design, because the bushings are hidden by other parts of the chain. The bushings can only be seen if you disassemble the chain. In a conventional chain, the two inner side plates are held together by tubular bushings, which are like hollow rivets. If you open a link of conventional chain, you can see the ends of the bushings flush with the outsides of the inner side plates. When the chain is assembled, the outer side plates hide the ends of the bushings.