The L2131BKP18 is a black PP plastic 38-400 child-resistant cap with pressure sensitive (PS) liner. Child-resistant caps are equipped with a locking mechanism that requires additional effort to open. To be designated as "child-resistant" these caps have undergone the requisite evaluations of a regulatory agency. This cap comes with a pressure sensitive (PS) liner. Pressure sensitive liners are made from foamed polystyrene. When the cap (with liner installed) is affixed to the bottle, the liner adheres to the bottle's landing/orifice. Before product can be dispensed, the lid must be removed, and the PS liner peeled off and discarded. This lid is made of polypropylene (PP), which means it has a high heat tolerance level and an excellent resistance to fatigue. This closure fits a container with a 38-400 neck finish. The first number refers to the diameter of the container's opening (in mm). The second number refers to the thread/style/depth of the closure's skirt. Please read 5 seals and how they make perfect liners for your container for more information on liner types. This lid is child-resistant making it ideal for packaging pills or other potentially hazardous products.