CD (compact disk) is a digital optical data storage format and was commercialized by Philips and Sony in 1982.  CDs are frequently used for audio content.  CDs are made from one disk of polycarbonate with metallized data layer just under the top labeling surface.  A laser follows a digital track recorded into the metallized layer and tracks are spaced 1.6um apart.  Care must be taken not to make impressions in the top labeling surface which can damage the data layer.  CDs measure 12cm (4.7-inches) diameter, 1.2mm (0.047-inches) thick, and have a 15mm diameter center hole.  CDs are becoming obsolete; car manufacturers have been phasing out CD players since before 2015.