The plastic bento boxes  using three types of plastic: polyethylene terephthalate (PET), polypropylene (PP), or a compound of PET and acrylonitrile butadiene styrene (ABS) called PET-A. A common practice is to use PP or PET-A for the bento box body and outer lid, and polyethelene for the flexible inner lids, or the main lids on small side boxes.