DOOR MARKER DOUBLE PLAIN. #DDM-OR. During a fire some rooms are searched 2 or 3 times while others are never searched. The double door marker simply snaps over both knobs to prevent latching. Door markers are manufactured in day-glow color so they are highly visible and they can easily be recognized by touch when in place. They are convenient to carry and work better than using chalk or tape to mark doors. These weigh 1 oz.