That's a good question, I'm glad you asked. In a nutshell, it is a pattern for a basic fitting shell with no seam allowances, and is used to make other patterns. They can be made by draping or drafting (followed up by fitting muslins to ensure that the pattern is accurate and correct), and once you have one that fits you can then alter it as needed to create other, more interesting styles. There are variations based on taste and use on what goes into a full sloper set; some people only do a two dart bodice and a skirt, sometimes the two dart front has a shoulder dart instead of a bust dart, et cetera - but the idea is still the same, that you have a "master" pattern to then alter to your heart's desire. There are many, many resources available on how to make one, including several commercial fitting shell patterns.
I made a dress size 8 sloper in school. It looks like this:
|Dress size 8 (or about a size 6 in today's off-the-rack sizes)|
This one was actually made for my drafting class, so it was not draped; the end results are the same, though. It's just two different methods for arriving with the same final product. This particular set includes a one dart bodice front, a two dart bodice front (with a bust dart), bodice back, straight skirt front and back, torso front and back, and a drafted sleeve with elbow dart. A bodice fits from shoulder to the waistline, a torso fits from shoulder to hip.
These pieces are cut from manilla card stock that is the same color and texture, if slightly lighter weight, than your average file folder. They have some very specific properties - they have notches at predetermined places, "drill" holes to mark pattern points inside the pattern, and a grain line as well as the name of the piece, the size, the maker, and the lengths of each section written along the perimeter. This is de rigeur for block patterns and helps assure that they are "true" and that they will work for specific body measurements.
|A closer image of the torso front|
|Detail of torso front, showing notches and drill holes|
These can be traced off quickly and then the copies become "working patterns" for new styles. They may get extended, reshaped, have the darts moved or converted into pleats or tucks or cowls, graded into larger or smaller sized - the sky's the limit. That is why I'm taking the time now to make one for my current dress form, and later I will make one that is specifically fitted for my body.
Also, I have arrived on a name for my dress form. She is to be called Marie - after the infamous Voodoo queen of New Orleans, Marie Laveau, for the simple reason that is she to have pins stuck in her like a poppet. Marie Antoinette was also discussed, since she was famously beheaded.