The B345A is a 4 oz amber PET plastic cosmo round bottle with 24-410 neck finish. The cosmo round, also called bullet round and imperial round, is characterized by its tall and slender build with steeply rounded shoulders. Polyethylene terephthalate (PET) is marked with the resin identification code of 1. PET is of the polyester family and has high clarity, good chemical resistance, high impact resistance, and medium rigidity. PET is also light-weight and is used for products containing essential oils. 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 item has a 24-410 neck finish. The first number refers to the diameter (in mm), and the second refers to the closure depth, style and thread. This container can be paired with a closure with a matching finish. This container is amber, a rich and vibrant orange-brown. In the case of glass, this amber color is made by mixing sulfur, iron and carbon into the base glass. Plastics are colored by using pigments. amber containers are shaded from light amber to almost black. This amber color provides some UV protection for light-sensitive products.