A cable guide is a fitting or part of a bicycle frame

Bicycle frame

A bicycle frame is the main component of a bicycle, on to which wheels and other components are fitted. The modern and most common frame design for an upright bicycle is based on the safety bicycle, and consists of two triangles, a main triangle and a paired rear triangle...

 which guides a piece of bare inner bowden cable

Bowden cable

A Bowden cable is a type of flexible cable used to transmit mechanical force or energy by the movement of an inner cable relative to a hollow outer cable housing...

 around a corner. Most multi-speed bicycles have cable guides to get the derailleur

Derailleur gears

Derailleur gears are a variable-ratio transmission system commonly used on bicycles, consisting of a chain, multiple sprockets of different sizes, and a mechanism to move the chain from one sprocket to another...

 cables past the bottom bracket

Bottom bracket

The bottom bracket on a bicycle connects the crankset to the bicycle and allows the crankset to rotate freely. It contains a spindle that the crankset attaches to, and the bearings that allow the spindle and cranks to rotate. The chainrings and pedals attach to the cranks...

.
Older derailleur bicycles used either brazed-on

Braze-on

A braze-on is the name for any number of parts of a bicycle which have been permanently attached to the frame. The term "braze-on" comes from when these parts would have been brazed on to steel frame bicycles...

 or clamp-on guides just above the bottom bracket, but newer bicycles have a guide under the bottom bracket.

Below the bottom bracket

Cable guides below the bottom bracket can be cheaper, just a piece of moulded plastic, and, for some bikes with very small chainrings

Crankset

The crankset or chainset , is the component of a bicycle drivetrain that converts the reciprocating motion of the rider's legs into rotational motion used to drive the chain, which in turn drives the rear wheel...

, eliminate interference between the rear derailleur cable and the bottom of the front derailleur cage. They also make for a cleaner appearance and easier to clean frame in the bottom bracket area. Poor lubrication of bottom-bracket cable guides is a common cause of autoshifting.

Above the bottom bracket

Cable guides above the bottom bracket are usually made of metal, causing more friction and wear on the cable, and is a more complex cable guide as it does not follow the shape of the bottom bracket shell.

They do allow use of a slightly shorter cable, tend to keep the cable more clean (as it is more protected from grit thrown up from the road), and the loop of housing at the rear derailleur does not need to bend quite as tightly, since the cable stop is on the top side of the chainstay, rather than beneath it. Despite the advantages this routing is almost exclusively found on older bikes.

Beside top of seat tube

Some bicycles use a cable guide on one side of the seat cluster for a rear cantilever brake

Bicycle brake systems

A bicycle brake is used to slow down or stop a bicycle. There have been various types of brake used throughout history, and several are still in use today...

cable, rather than use a short length of housing between two housing stops.