A plain seam is a basic sewing technique used to join two pieces of fabric together. It is created by placing the fabric pieces right sides together and sewing along the edge, typically with a straight stitch. This type of seam is commonly used in various sewing projects, from clothing to home decor. Plain seams can be finished with techniques like zigzag stitching or serging to prevent fraying. They are versatile and can be adjusted in width and length depending on the fabric type and desired strength. Properly sewn plain seams provide a clean and professional look to any garment.