Antonio Marras, RST14, New York.
When pattern making any garment that crosses over from the shoulder to the arm, you need to consider the placement of seam lines and have an understanding of how the seam line placement will affect the shaping of patterns. The position of the armhole seam line, and the amount of volume in the pattern will have a large part to play in how the silhouette is created, and in how much movement there is in the armhole for the person wearing the garment.
Set In Sleeve
When the vertical seam line is at the shoulder point then the bodice pattern will not need to do any shaping for the curve of the top of the shoulder. This means that you will need to use the shape and volume in the sleeve head to shape around the curve at the top of the shoulder.
This type of sleeve is usually referred to as a set in sleeve. When constructing this seam you will need to sew the bodice pieces first, and then ease the extra volume in around the top of the sleeve head.
Examples of this type of sleeve could be seen in the Antonio Marras Resort 2014 collection:
Slight Drop Shoulder
If you are creating a drop shoulder design, then the vertical seam for the armhole will be created to go somewhere between the shoulder point and the top of the arm. In fitted styles this will mean that the shoulder curve will now need to be split between the bodice pattern and the sleeve pattern, so that they both share the job of accommodating for the shoulder curve.
An example of this type of slightly dropped seam could be seen in the Antonio Marras Resort 2014 collection:
Oversized Dropped Sleeve
If your silhouette is not fitted, but you are still aiming for a dropped shoulder, then sometimes you will just need enough volume in the pattern to allow the bodice and sleeve to drape over the shoulder curve. In this case, you will not need the pattern to be shaped for the shoulder you just make the pattern big enough so that there is enough fabric to drape around the shoulder and armhole area.
As an example, this sort of pattern change may mean that you will need to enlarge the size of the armhole on your bodice. This will mean both extending the shoulder line and also making the underarm lower to give the arm more space to move.
The sleeve pattern would then be adjusted to fit the new bodice armhole, often resulting in a sleeve head curve that is lower and wider. The new dropped sleeve will generally be lower and wider than a standard set in sleeve because the dropped sleeve will not have to reach up and over the shoulder curve anymore.
An example of this type of dropped sleeve can be seen in this image below:
Once you have moved the silhouette away from the body you can also vary the way that you create shaping and panels, because you do not need to strictly adhere to a traditional fitted armhole shape. Below are some other examples of how you can think about making room for the shoulder and arm without using traditional bodice and sleeve shapes:
Images from Vogue.co.uk»
Diagrams from The Cutting Class»