A buttonhole is basically an opening in the fabric which the button can slip through. And while you can cut a slit and use it that way, the fabric will wear and fray, destroying the buttonhole. Even non-fraying fabrics like felt will stretch and show wear with a lot of use. Binding the fabric edge protects the fabric and creates a strong opening for the button to pass through again and again. Additionally, buttonhole stitch does more than just wrap thread over the edge as an overcast stitch would. Each stitch makes a knot at the edge of the opening, locking the stitches together. Modern sewing machines usually have a setting that automatically makes a buttonhole for you, and while this is helpful, it’s not always the best option. Even if you have a machine that could do this for you, you might choose a hand-sewn buttonhole so that you have more control over the shape and size. If you’re making historically accurate garments as costumes, the buttonholes would never have been made on a sewing machine. Hand finishing a buttonhole is also a way to give your handmade garments a well-tailored couture element. Sewing patterns will tell you how to prepare an area for installing a buttonhole, so be sure to follow those instructions. Buttonholes are usually reinforced with interfacing and/or a double layer of fabric. You can sew your buttonhole stitch with a doubled strand of good quality sewing thread that matches the fabric, but for better visibility, this example shows contrasting embroidery thread. Next, make small running stitches around the buttonhole space. You can make this rectangular like the sample or you can create a different shape like a keyhole. The running stitch defines the edges of the buttonhole which makes it easier to keep the stitches even while also preventing the fabric from fraying under the stitching. Keep the thread attached so it is ready to continue after the next step. Insert the needle through the fabric just outside the running stitch, then bring the needle back up through the buttonhole opening. Pass the working thread behind the tip of the needle. Pull the needle through. Insert the needle just outside the line of running stitch and bring it back up through the opening. Keep the working thread behind the needle, then pull the needle through to complete the stitch. When you need to end a thread and start a new one, go down through the fabric as you do to start each stitch. Secure the thread on the back. Anchor a new thread with a knot and a small stitch on the back, then bring the needle up through the opening, catching the last stitch so the new thread locks with the previous one.
