The parts of a bicycle chain are, from the inside, to the outside of the chain; the roller, the roller link plate (also called the "inner link plate" or "inner plate"), the pin link plate (also called the "outer link plate" or "outer plate") and the pin (or rivet). The roller is the part that revolves around the pin or rivet and makes the greatest contact with your chainring and cogs, pulling on them to propel your bike. The roller rests between roller link plates with the pin through all the plates and the roller. Flanking the roller is the roller link plates. These hold between them two rollers with a rivet passing through at each end. On the outside of the roller link plates are two pin link plates. The pin link plates join two of the roller link units. A chain link is two of the roller/link pin assemblies and two plates either roller or link plates. Chains have become narrower in the last few years. Manufacturers have discovered more is better and bigger is best, so the number gears on bikes has increased from 10 to 24. The way this number has been increased is to have the width of the cogs reduced so that the spacing between the rear cogs could be made narrower, fitting more cogs in the same space. Finally, in 1989 Shimano widened the rear axle on mountain bikes to 135mm from the 130mm that had been the industry standard for over a decade. With the 135mm rear and the 4.8mm on center spacing uses for rear cogs, all we need to is adjust the hub flanges a tiny bit and a 10 cog rear cassette is possible. Chain wear is measured in two ways. The first is known as deflection. With a new chain, if you were to place it on a table so the link pins were horizontal with the table, and grabbing both ends, flex the chain so that it formed an arc shape, the distance between where a straight line forms at the ends and the top of the arc is called "deflection". The second measurement is known as "elongation", or chain stretch. New chains have all their parts joined with the same element of precision, but over time the chain gets stretched. New chains have very little deflection or elongation. As chains wear, the grinding of their parts against one another scores and cuts into adjoining chain parts. All of these gaps form a sloppiness that must be pulled out of the chain with each crank revolution. A chain that's worn, continues to wear out at a faster and faster rate.