Need description of Default Value format

We need a description of the default value field for columns. In SSMS, what is the correct way to specify an empty string? I've read forums with conflicting results, saying that entering two apostrophes results in the apostrophies being added literally, and if they set the default value as an empty string via SQL, then SSMS shows the default value as two apostrophies contained within parenthesis. Now, if I use either of those, they both end up as empty strings, so it's confusing, and there's