Thanks to all who've been responding!

Barbara, I don't know much about blues, and I'm not sure what was the thing I meant that you're asking about.

The melodic minor that Bruce mentioned is the one with raised 6th and 7th ascending and lowered descending. A look at "Greensleeves" and (I believe) many classical pieces will show that that description is oversimplified. The raised 7th (leading tone) is used to create an expectation that the tonic will follow--and usually it does follow. It's also used as part of the dominant chord. The raised 6th is used when the raised 7th follows or precedes it. Or when it sounds good--the real rule.