A skirt guard or coatguard is a device fitted over the rear wheel of 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....

 to prevent a long skirt

Skirt

A skirt is a tube- or cone-shaped garment that hangs from the waist and covers all or part of the legs.In the western world, skirts are usually considered women's clothing. However, there are exceptions...

, coat or other trailing clothes or luggage catching in the wheel, or in the gap between the rim and the brakes.

Skirt Guards are common on bicycles in continental Europe, where the bicycle is commonly ridden in smart-casual or formal clothing; but very uncommon in English speaking countries where many cyclists wear specialised cycling gear. It is often paired with a chain guard to keep the rider's clothing clean. They are often used on utility bicycle

Utility bicycle

A utility bicycle is a bicycle designed for practical transportation, as opposed to bicycles which are primarily designed for recreation and competition, such as touring bicycles, racing bicycles, sport/training bicycles, and mountain bicycles. The vast majority of bicycles can be found in the...

s.

There are several forms, for instance:

• A length of string may be threaded through holes in the rear mudguard, running radially between the mudguard and the rear dropout
  Dropout (bicycle part)
  A bicycle dropout is a type of fork end that allows the rear wheel to be removed without first derailing the chain. The more inclusive term fork end refers to a slot in a frame or fork where the axle of the wheel is attached....
  
  .
• A lightweight metal mesh covering the upper half of the rear wheel.
• A fabric mesh or net stretched over the same area.
• A flexible canvas secured over the same area.
• A solid, rigid metal or plastic plate, which can be smaller than a canvas piece.

Large, solid surfaces have the disadvantage of air resistance, which increases the risk of being blown sideways by gusty sidewinds. Air-permeable designs such as mesh and string provide a more stable ride, but can be more difficult to keep clean. A pair of small plastic sheets gives moderate air resistance and a smooth surface which is easier to keep clean.