The J538WH is a 4 oz white PP plastic double wall straight base jar with 70-400 neck finish. Double wall jars are 2-piece containers: the shell and the liner. It is called "double wall" because the two containers nested inside of each other create two walls. This jar has a straight base. This jar is made of polypropylene (PP). PP is light-weight, durable, flexible, and highly resistant to fatigue and heat. It has a high shatter-resistance rating. This jar is typically used for lotion, cream, serum, gel, putty, clay, hair products, skin products, cleanser, and other personal care products. This jar has a 70-400 neck finish, and will fit closures with the same finish. The first number is the diameter of the container's opening (in mm). The second number refers to the thread/style/depth of the closure's skirt.