Many commercial patterns for shirts, blouses, jackets, and coats are designed with a one-piece sleeve. However, if you want to add details such as cuffs with plackets, or vents, you need a back sleeve seam.
In Threads #204 (Aug./Sept. 2019), Contributing Editor Louise Cutting shares two clever methods to add interest to sleeves, using a back-sleeve seam. One example is a faux tailored vent, which is quick to sew and looks like a functional sleeve vent. The other example is a placket placed at the back sleeve seam; with this method, you don’t have to slash and bind a placket opening in a one-piece sleeve.
If your garment pattern has a one-piece sleeve, you can add a seam to accommodate these vent techniques. Simply divide the armscye seamline into approximate thirds; do the same for the hemline. Draw a line connecting the marks that are one-third from the sleeve back seamline. The illustration below shows how to divide and mark the pattern and then draw the line.
Cut the pattern apart along this line and add a 5/8-inch-wide seam allowance to the new seamline on each piece. Then adapt the two-piece pattern to include a faux vent or an easy-to-sew placket.