1. The call numbers are arranged alphabetically by the first line.P is shelved before PR
2. Call numbers with matching first lines are then arranged numerically by the whole number on the second line.PR is shelved before PR is shelved before PR
98 671 2313
3. Call numbers are then arranged alphabetically and then numerically, by the third line. The numbers in the third line are decimals. This means that:PR is shelved before PR
*This is also true of the fourth line if it is a letter number/combination. It may also be a year, in which case it is arranged numerically/chronologically, lowest to highest.