Chainline is the angle of a bicycle chain

Bicycle chain

A bicycle chain is a roller chain that transfers power from the pedals to the drive-wheel of a bicycle, thus propelling it. Most bicycle chains are made from plain carbon or alloy steel, but some are nickel-plated to prevent rust, or simply for aesthetics. Nickel also confers a measure of...

 relative to the centerline of the bike's frame. A bicycle

Bicycle

A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....

 is said to have perfect chainline if the chain is parallel to the frame, which means that the rear sprocket

Sprocket

A sprocket or sprocket-wheel is a profiled wheel with teeth, cogs, or even sprockets that mesh with a chain, track or other perforated or indented material. The name 'sprocket' applies generally to any wheel upon which are radial projections that engage a chain passing over it...

 is directly behind the front sprocket. Chainline also refers to the distance between a sprocket and the centerline of the bike.

Bicycles with straighter chainline are more efficient due to the frictional losses incurred by running the chain straight around both sprockets but at an angle between them. This is the main reason that a single-speed bicycle

Single-speed bicycle

A single-speed bicycle is a type of bicycle with a single gear ratio. These bicycles are without derailleur gears, hub gearing or other methods for varying the gear ratio of the bicycle....

 is more efficient than a derailleur geared bicycle which is shifted into an identical gear combination. Single-speed bicycles must have the straightest possible chainline in order to prevent the chain from falling off. This is particularly important for fixed-gear bicycles, because chain derailment can cause such a bike's rear wheel to lock.