The B231BK is a 16 oz black PET plastic squat boston round bottle with 24-410 neck finish. The squat boston round bottle has rounded shoulders and rounded base, but has a higher diameter to height ratio than a typical boston round. 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.