A Russian birch bark letter (14th century).

Birch bark or birchbark is usually the bark of the paper birch tree or sometimes of similar trees such as gray (wire) birch.

Birch bark is useful because it is strong and water-resistant. It is similar in some ways to cardboard. And can be easily cut, bent and sewn. Because of this it has been very valuable for building, crafting, and writing material ever since ancient times. Birch bark also contains substances that are used in medicines and chemicals.