these photoluminescent anti-slip step markers get installed on the very left and very right of each step in a staircase to highlight the path of escape. Always mark both sides of a step, requiring one left and one right marker. The markers absorb and store energy from ambient light and are immediately visible in a sudden black-out or underneath smoke. The vinyl step markers have an anti-slip surface and a thin adhesive backing. Important: install on wet-cleaned, dust-free, dry smooth step surfaces. Nyc mea listed by new york city department of buildings.