I'm glad to see that a better reference has already been given. Saves me the trouble of looking for it.

An interesting thing (maybe already mentioned in the discussion - I'm a bit short on time) is that deliimiting identifiers with [square brackets] is totally non-standard. As far as I know, only SQL Server and Access support this. The ANSI standard and all complying products use "double quotes" to delimit identifiers.

I think the prime reason for the QUOTED_IDENTIFIER option is to supply backward compatibility. Old versions of SQL Server allowed both "double quotes" and 'single quotes' to delimit string constants, and this was sometimes used to enabl easier specification of string constants that include a single quote / apastrophe (so instead of 'O''Brian', you'd write "O'Brian").