Rack Marks and Airbell Markings on Motion Picture Film
J. I. Crabtree, C. E. Ives · Transactions of the Society of Motion Picture Engineers · 1926
When developing motion picture film by the rack and tank system it is very difficult to secure uniform development throughout the entire length of the film. Unless special precautions are taken, more development occurs at the top and bottom of the rack where the film passes over the end slats or bars than along the sides, so that bands of greater density occur at intervals corresponding with the height of the rack, which cause an objectionable flicker when the film is projected. These dark markings are termed “rack marks.”