Why are dictionaries in alphabetical order?
We mention in our article about dictionaries over time that the earliest form of the English dictionary evolved from glossaries that listed Latin words and their Old English translations. While some of these glossaries listed words alphabetically, others did so thematically. When monolingual and bilingual dictionary-making came into greater prominence in the 17th, 18th, and 19th centuries, lexicographers carried over alphabetical ordering, most likely so that people would have an easy way to find the word and/or definition they are looking for.
There are, however, certain types of dictionaries for which using alphabetical word ordering is either impractical or just a roadmap to getting to the right word or meaning. In cases where a person is looking for a word, but only knows its definition or related words, reference books like reverse dictionaries and the Roget-style thesaurus categorize words by their key concept or semantic class first to help people find the exact word. While the general themes or meanings are typically sorted alphabetically, it’s not uncommon to find the words that belong to these classifications sorted differently.
Additionally, languages that have character-based writing systems, such as Chinese or Korean, have adopted many different methods of ordering logograms. For instance, the system used in the Oxford Chinese Dictionary can be as involved as ordering by pronunciation, tone, stroke number, and first stroke in the character.
Source: Questions of English, compiled and edited by Jeremy Marshall and Fred McDonald (Oxford University Press, 1996)
See other FAQs about dictionaries.