A seam gauge is a special tool for sewing which is designed to ensure that seams and hems are even, creating a garment which fits properly and feels comfortable. In addition to ensuring that a garment is constructed well, this tool also helps to create a more professional, polished look in the finished piece, and it helps to keep seams sound, reducing the risk of unraveling or gaping open.
Basically, a seam gauge is a ruler, allowing people to measure fabric, but it includes a special slider which can be used to mark a desired measurement. When using the gauge, sewers can set the marker to a specific point and use it as a quick visual point of reference while pressing, pinning, or sewing a seam. Eventually, the marker may bend or start to slide, in which case it or the entire gauge will need to be replaced.
Most sewing patterns include what is known as a “seam allowance.” The seam allowance ensures that once fabric has been cut to a specific size, there will still be room to sew it together. Typically, a pattern specifies its seam allowance, so that sewers know how much leeway they have, and some people leave an extra-large seam allowance, just to be safe. When you know what your seam allowance is, you can set your seam gauge accordingly, ensuring that the seams are the proper width.
In addition to being used to measure seams, this tool can also be used to keep darts even, by carefully measuring each dart. This is extremely important, because unevenly sewn darts can cause a whole garment to look uneven, destroying the overall look of the piece and often interrupting the fall of the fabric along the body. On visible seams and hems, a seam gauge is used to keep the sewing even and straight, so that the garment looks attractive when it is finished.
Seam gauges can be made from plastic or metal, and the moving part typically has a clip or screw to keep it in position. Typically, they are not painted, to ensure that nothing rubs off onto fabric as it is handled and sewn, and some gauges are hollow in the middle, allowing people to use them as a guide for tailor's chalk.