A growing number of custom decks on today’s marked playing card market are advertised as marked, which means they have secret markings that allow you to know the value or suit of a specific card in advance. These markings are hidden by various methods and can be accessed using different systems. Some are so obvious they can be seen by anyone, but others require a more advanced reading system such as a camera, sensor, or contact lens. Some of them are so obvious that they are only visible to people who are familiar with them.
There are a few subtle methods of marking cards in such a way that they can be read without suspicion, such as tinting (subtly tinting different parts of a small detail on the card back, such as a head for an ace or left wing for a king). Some systems involve blocking out specific parts of a card design or adding gaff on the back – such as cutting out the bird from a flower detail. Some systems rely upon chemical solutions like juice dust or daubing which can be used on the fly to mark a card deck while it is in use.
The quality of the marked deck is important. A deck of high quality should be free from stains, scratches and other damage. The marks on the deck should be visible and readable, but not too bright.