"A shoe tree is a device approximating the shape of a foot that is placed inside a shoe to preserve its shape, stop it from developing ...
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creases and thereby extend the life of the shoe. It is a reusable alternative to wadded rags or newspapers.
Perhaps more important than maintaining the shape, shoe trees also play a crucial part of wicking away leather-damaging moisture from the inside. This is especially important when shoes are worn without socks, as the sweat absorbed by the leather can lead to lining rot.
Higher quality shoe trees are made from solid wood, usually cedar, which helps control odor and absorb moisture." - (en.wikipedia.org 15.03.2020)