Single wall jars are normal 1-piece jars. They're called "single wall jars" to distinguish them from the 2-piece double wall jars. 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.