The J139CL is a 4 oz clear PET plastic single wall jar with 70-400 neck finish. 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 polyethylene terephthalate (PET). PET is of the polyester family and is semi-rigid, and very light-weight. It has high clarity, is strong and impact-resistant. It has a good chemical resistance. Avoid using PET containers to store products that contain essential oils greater than 5% or fragrance oils greater than 15%. If your oil percentages exceed these respective levels, please consider using glass. 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.