The existence, or not, of a difference in meaning between "inquiry" and "enquiry" is surely an area of the English language that needs urgent, authoritative attention from the likes of the OED, Fowlers and Chambers.
As there seems to be a need to distinguish between an act of formal investigation and a simple question, I cannot see any alternative but to clearly associate the former with the noun "inquiry" and the latter with the noun "enquiry". Another question immediatly turns up. Can one "inquire" about something?
Another alternative might be to consider "inquiry" a synonym of "investigation", in which case "inquiry" might be redundant. "Enquiry", however, does not seem to have any perfect synonyms as it seems to indicate the particular way in which the question was asked. The "enquiry" thus symbolizes the entire act - and its sub-parts - involved in requesting specific information. This, however is not what the OED states when it refers to "inquiry" from "enquiry", thus seemingly making "inquiry" the preferred spelling of the same word.
Due to the fact that there is no perfect synonym for "enquiry" (i.e. another single word cannot be plugged into a sentence in place of it and maintain the same sense, such as "please respond to my question"), I consider "enquiry" to be indispensable, while "inquiry" can be satisfactorily replaced by "investigation"
Why is the OED so lackadaisical about this?
Frank —This comment was unsigned.
- Probably related in the fact that enquiry isn't a real word in American English. --Connel MacKenzie 03:02, 24 January 2008 (UTC)