Turned covers refer to envelopes that have been turned inside out and reused. Often times this was done to reply to the sender of the original letter. A turned cover is actually two pieces of postal history on a single cover. One side of the envelope has stamp(s), postal marking(s), and so forth for one usage. And the other side of the envelope has stamp(s), postal marking(s), and so forth for the second trip through the mail system.
Why was this done? The practice was frequently used by people in the South during the Civil War. The south experienced a paper shortage during the war. Paper products, like envelopes, were expensive and hard to find. Someone came up with the idea of turning an envelope inside out to reuse it. What’s the phrase? Adversity sparks ingenuity?
There are other known instances of turned covers. Some are gimmicks. Some are philatelic creations. These usages are not particularly valuable. Turned covers are not rare. Common covers sell in the tens or few hundreds of dollars. Covers with rare stamps, unusual postal markings, or other extraordinary factors sell for much more.