A shoe tree is a device approximating the shape of a foot that is placed inside a shoe

Shoe

A shoe is an item of footwear intended to protect and comfort the human foot while doing various activities. Shoes are also used as an item of decoration. The design of shoes has varied enormously through time and from culture to culture, with appearance originally being tied to function...

 to preserve its shape, stop it from developing creases and thereby extend the life of the shoe.

Higher quality shoe trees are made from solid wood, usually cedar

Cedar wood

Cedar wood comes from several different trees that grow in different parts of the world, and may have different uses.* California incense-cedar, from Calocedrus decurrens, is the primary type of wood used for making pencils...

 which helps control odor

Odor

An odor or odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors...

 and absorb moisture

Moisture

Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...

. Wooden shoe trees are often made with two or three pieces of solid wood with a solid metal stem inserted between the heel piece and the single or double toe piece/s which have a spring action so the trees fit more snugly into the shoes. They often have handles or brass knobs at the heel piece for the fingers to grasp and pull out the trees from the shoe when removing them.

Shoe trees may also be made of plastic

Plastic

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

, with or without a coiled metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 spring

Spring (device)

A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

stem; these are typically cheaper, lighter, and are better suited for travelling.