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Pins & Ramps
Modern chainrings intended for derailer use commonly have special features on the sides to improve upshifting . These are pins, usually riveted to the sides of the larger chainrings, and ramps cut or stamped into the inboard sides of the chainrings.
These pins and ramps help catch the chain to climb from a smaller to a larger chainring. The smallest chainring will usually not have these features, because you never upshift to the smallest ring.
The protruding rivets of older chains would catch in the space between chainwheel teeth. Modern chains for narrow sprocket spacing have rivets that do not protrude, and so the pins on the chainwheels take on the task of lifting the chain.
Steel pins are used in aluminum chainrings for improved wear resistance. The pins do the actual lifting of the chain, and take the most stress. The ramps just provide a smooth path for the chain to climb once it has been lifted by the pins.
Hyperglide-type rear sprockets have similar ramps, but don't need pins, because the sprockets are of hardened steel.
These features help improve shifting, and make rider skill less important in shifting, but none of these features is actually essential to the functioning of any system.
