Hold-to-light (HTL) postcards come in two types: die-cut and transparencies. Die-cut HTL cards have a design cut out of the card and that part is covered with thinner paper. When the card is held to a light, it comes through the thinner paper with a design.
A simple example would be a stained glass window with the windows cut out and covered with paper.
Transparencies are not cut out but are made of three different layers of paper. The middle layer has a hidden design which is usually related to the front design. Objects, characters, colors, or scenes appear magically when the postcard is held in front of a strong light.
An example would be a secret message that appears only in strong light.We have a number of each type of HTL card in our current auction at Cherryland Auctions.