Refers to the number of gear combinations that a bicycles drivetrain is capable of. When refering to how many speeds a bike has it is most common to multiple the number of chainrings on the crankset by how many sprockets make up the cogset.
A crankset with 3 chainrings paired with a rear cogset with 9 sprockets would be a 27speed bike. However; it can also be common to simply refer to how many sprockets make up the cogset because that is a better determiner of what chain will be compatible with the system.
For example: If you have 3 chainrings on the front and 9 sprockets on the cogset many bike shops will simply refer to it as a 9 speed system because only a 9 speed chain will be compatible with it.
In recent years, many bicycle manufacturers have started transitioning to only using 1 chainring on the crankset and expanding the range or number of speeds of the rear cog. This allows the drivetrain to be simplified, reduce weight and be cheaper to service since there is no front derailleur to be adjusted. The speed of these systems is a prime example of why many people simply refer to how many sprockets are on the cogset because 1 chainring multiplied by 11 sprockets is still 11.
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