It took me years and years of sewing before I finally learned what a princess seam was.
Princess seams are flattering to almost all figure types. They will enhance your waist, if you have one, or give you a waist if you don’t. They add another vertical line in your garment which elongates the figure (most of us can use all the vertical lines we can get!).
Here is a line drawing of Petite Plus Princess Seamed Blazer – 202.
Notice the seam between the Center Front and the side seam on the jacket front and the seam between the Center Back and the side seam on the back? It is slightly curved at the top end.
This seam is the “princess” seam.
It follows the general hour-glass curve that is the ideal womanly figure type every woman wants to achieve.
This is one type of Princess Seam. Because it intersects with the armscye (or armhole) seam at the top, I will call it the arm-hole type of princess seam.
Here are some line drawings from Butterick dress pattern number 3114 with a shoulder type of princess seam. The intersection of the princess seam occurs not at the arm-hole but at the shoulder seam.
The slimming, hour-glass effect of the princess seam is basically the same for both types.
With just a little altering, it is possible to change an arm-hole type of princess seam to a shoulder type or vice-versa just by re-drawing the princess seam and slightly changing the connecting pieces to adjust seam allowances.
On either type of princess seam, the actual princess seam is meant to run up-and-down directly across the apex of the bust. In a well-fitted garment, it will. For many women this means adjusting the amount of curve in the seam as their bust often is placed to one side or the other or above or below the “curviest” part of the seam (the widest part of the bump on the side piece).
Adjusting a princess seamed pattern is a little bit challenging but can be done and, if done correctly, is an incredibly attractive look.
The best advice I’ve found about altering these seams is to pin-fit the pattern and, while on yourself or your dressform, unpin part of the princess seam to allow it to spread if more fabric is needed or pin snugger if less is needed. If it is too tight, unpinning that princess seam, just over the bust, actually shows you whether more fabric is needed on the side front or on the front piece so you can add it where it is needed. Make sure you are wearing a great bra so your bust is held within the width of your body frame.
This is a better method than just adding half of the total amount needed to both the side front and the front piece. For example, I find that I usually need very little added to the front piece and much more added to the side front. If I just added half the total amount to the side piece and half to the front piece, the seam would not lie directly across my bust apex.
As opposed to a princess seam, here is a line drawing of Buttrick pattern #5065 showing the jacket shaping done with darts and tucks instead.