Double envelope is a physical security measure for sending classified documents by mail. The document is placed in an inner envelope bearing the classification marking, which is then inserted into a neutral outer envelope.
The outer envelope shows only the recipient address and no indication of the content or classification. This prevents anyone along the transport chain from learning that confidential material is being sent. Damage to the inner envelope reveals tampering attempts. ISO 27001 Annex A (A.7.10) requires the protection of information during transport — the double-envelope method is one of the oldest and simplest measures for this purpose.