In the Dewey Summaries, I notice 777 is an unassigned number and 789 is optional. What's the difference between an unassigned number and an optional number?

The main difference is that unassigned numbers are vacant, while optional numbers can accommodate local emphasis (if such use is warranted).

An unassigned number is just that: a number which has not been assigned any meaning in Dewey. Some unassigned numbers are used for options, some have been assigned meanings in previous editions in Dewey, and some have never been assigned a meaning.

In the print edition and in WebDewey, unassigned numbers at the second level (there's only one, 040) and at the third level are printed or displayed, but there is an unlimited number of unassigned numbers at other levels. For example, 994.9 is an unassigned number, and has never been given a meaning (and for that reason, has never been published in Dewey); 192.7 is an unassigned number, and was last used in DDC 14 (published in 1942, and not to be confused with Abridged Edition 14). Classifiers are encouraged not to use unassigned numbers for private or local developments, because an unassigned number could be used at any time if needed. Unassigned numbers appear in brackets, e.g., [777].

Some devices are required to enable the DDC to serve needs beyond those represented in the standard English-language edition, and the optional number is such a device. An optional number can serve as an alternative to standard notation provided in the schedules and tables to give emphasis to an aspect in a library's collection that's not given preferred treatment in the standard notation. In some cases, options are also suggested to provide shorter notation for the aspect. Most of the time, the responsibility for implementing an option rests with the local library, and libraries should weigh the value of using an option against the loss in interoperability of numbers. Optional numbers appear in parentheses, e.g., (789).

Some numbers are permanently unassigned and have been assigned an optional meaning. These numbers, like other optional numbers, appear in parentheses and have the caption "Permanently unassigned." There is one exception to the latter--298, the one permanently unassigned number that appears in the DDC Summaries, has the caption "Optional number" to save space and to indicate its special use.

Last revised: 21 April 2005