Mirror the alphabet (A↔Z, B↔Y…) to encode or decode. The same operation reverses itself.
Atbash maps each letter to its mirror in the alphabet: A↔Z, B↔Y, C↔X, and so on. It originated as a Hebrew substitution cipher.
Run the same text through Atbash again — the cipher is its own inverse, so encoding and decoding are the same operation.
Yes. Only letters are mirrored; digits, spaces and punctuation pass through unchanged.