What are the regulations for using the skins of edible and non-edible animals, whether tanned or untanned?

Animal skins are divided into three categories:

1. Pure: The skins of animals whose meat is permissible to eat and that have been ritually slaughtered, whether tanned or not. 2. Always impure: The skins of animals whose meat is not permissible to eat (such as dogs, wolves, and pigs), whether they were slaughtered, died naturally, or were tanned or not. The preponderant opinion is that tanning does not purify them. 3. Purified by tanning: The skins of animals whose meat is permissible to eat if they died without ritual slaughter. These are impure before tanning and become pure after it.