There are many designs and numerous methods used to get access to the hidden message. Some envelopes look obviously gimmicked, others do not allow examination. 

The eXtRA-eYe Peek Envelope is a small black envelope (approx 9cm x 6cm), totally opaque front and back. Yet in performance - it allows the performer a full view of the written message that's placed inside. What's more, it can be thoroughly examined. 

In a typical performance, you have a spectator write a freely chosen number on a card, you then place the card inside the envelope, seal it down and place it on the table turning it over as you do, so as to show it to be opaque on both sides. Having done this, you would now have had a full vision of what is written on the card that is inside the envelope. You can even hand the envelope to the spectator and let them closely inspect it. It looks and feels like an ordinary envelope and their written on card is inside. Having revealed or used the information in whatever way you wish, simply cut or tear the envelope and remove their card.

You can view video footage on this page to see the eXtRA-eYe envelpe in use. 

Sold in packs of 20 envelopes