Possible Duplicate:
A special use of “should”?

I have received an email which was starting wih this:

Should you have any questions, please call at this number...

So could somebody explain the grammar form of "should" here and why it is preferred over "if".

merged by RegDwigнt Apr 29 '12 at 11:44

This question was merged with A special use of “should”? because it is an exact duplicate of that question.