Quote Originally Posted by mimitabby
but why are we calling a chainring or gears a crank? I am confused.
A crank consists of chainrings, spider, and pedal arms. Because you need different spider arms with a different bolt circle diameter for a triple, double, or compact double, the chainrings aren't interchangeable. And the right pedal arm attaches to the spider. So you need the whole shooting match to switch from one to the other, ie, you have to replace the crank.