Here I am, again, after 3 months away, to fill another tutorial need. If you want something done right...
When you're sewing thick fabrics, or several layers, or just sewing a small enough project that every detail matters, you may want to grade your seams. It's simple to do.
On a garment with a lining or facing, when all the layers of a seam allowance are cut to the same width, they can leave a bulky lump along the edge of the garment. I've sewn together three random fabric remnants to illustrate:
I've run the iron along the edge of the seam allowance to make it show up better for the picture. You can see how thick the edge is, and how sudden the transition is from the seam allowance to the main expanse of fabric.
To ease the transition from seam allowance to main expanse, trim each layer of the seam allowance to a different width. This photo shows the innermost layer trimmed to about an eighth of an inch from the stitching.
And here you can see the second layer of the seam allowance trimmed to a width between the outer fabric and the inner fabric. See how the edges of each layer are a different width from the seam?
On a lined garment, you can usually trim quite close to the stitching without fear of it raveling enough to come apart at the seam. If you're really worried that the fabric won't hold up, you might try running a bead of Fray Check along the cut edge after you trim the allowance. Make sure the Fray Check dries before you fold the seam closed again - it could mark the garment if it bleeds through to the outside.
Here is the same seam seen from the outside after grading. Notice how much smoother the transition is from edge of the garment to main expanse of fabric. It's best if the widest seam allowance is the one closest to the outside of the garment, for the smoothest finish possible. It's just one trick, of many, for a more professional finish on your home sewn garments.