Used in both sewing and knitting, an invisible seam is one that tends to be less bulky and for the most part not apparent to the eye. In both sewing and knitting, the line on which the seam is sewn is referred to as the stitching line or the seam line. Whether in sewing or knitting, the invisible seam is flat, and there are no lumps or apparent bulges along the line of stitching. An invisible seam is not a decorative seam.
In sewing, the invisible seam is considered a basic seam on which many other seam types are based or built upon. This type of seam is used with a large array of fabric types. When creating a clothing garment, this seam will be used when the garment is to be lined. This allows for a more comfortable fit, as the seam will create less bulk and will lie flat against the body of the person wearing it.
When sewing, using a fine, curved needle will aid in helping achieve the highest quality invisible seam. Often the ladder stitch is used to create this type of seam when sewing on material that has nap or fur. This stitch will also work on traditionally woven napless fabric. Once the stitch is complete, tighten the thread and the seam should virtually disappear. If stitching stuffed animals or quilted patterned toys, hand quilting thread is often used for keeping a seam firmly in place.
When knitting, the invisible seam gives the deceptive appearance that the knitted stitches are continuous. The most important element in achieving this type of seam within knitting is to keep consistently even tension along the seam as stitches are made. If tension is irregular, the seam will no longer appear invisible but instead will be raised and puckered.
In knitting, the invisible seam is considered by many to be the most professional joining method. It is, however, not a decorative method for joining two pieces. When invisible seaming, the mattress stitch is most commonly used. To achieve a mattress stitch, which will prove to be an invisible stitch, the key is to keep the seam elastic throughout. Another important detail of this type of stitch is to catch horizontal threads on each side. The seam can neither be left to be too loose, nor can it be pulled too taut. Otherwise, the seam will be visible, which defeats the entire purpose of the invisible seam.