A reference code for a specific glass type that is represented by three digits, then a slash, then another three digits. It is always present in technical prints to indicate the specifications of the glass substrate used. The first three digits refer to the three numbers after the decimal in the glass' index of refraction. The second three digits refer to the first three significant digits in the glass' Abbe number. For instance, BK7, which has an index of refraction of 1.517 and an Abbe number of 64.2, has a glass code of 517/642.