Filmguard Polyester Twin & Triple Checks are designed to make it easy to correlate processed film. Twin & Triple checks are easy to read, and made to withstand photo process temperatures without detaching from film.
These checks are used to keep "tabs" on who belongs to the film and how it is processed. One check goes on the film envelope while the other goes on the film itself. In this way, film and envelope can be reunited when processing is complete.