A seam is formed when stitches permanently join layers of fabric. It may provide structural strength, remain almost invisible, or become a deliberate design detail. The correct seam depends on material, stress, shape, and the desired inside finish.
Parts of a seam
The seamline is where the construction stitching sits. The seam allowance lies between that line and the raw cut edge. On a domestic sewing machine, the upper thread and bobbin thread normally interlock inside the fabric to form a lockstitch.
Balanced thread tension places that interlocking point between the layers. Loops, puckering, or bobbin thread pulled to the top indicate an issue with tension, threading, or the needle-and-thread combination.
Common seam constructions
- Plain seam: sewn right sides together, with allowances pressed open.
- Closed seam: both allowances are finished together and pressed to one side.
- French seam: a second line of stitching encloses the raw edges.
- Flat-felled seam: one allowance wraps the other and is topstitched down, common on jeans.
- Overlock seam: depending on its setup, trims, joins, and finishes the edge in one operation.
Strength and stretch
A straight stitch suits many stable woven fabrics. Knits require a stretch-capable stitch or overlock seam so the thread does not break during movement. High-stress areas may be reinforced with a second seam, shorter stitch, or bar tack. Extremely short stitches can also weaken delicate fabric by perforating it.
Accurate construction
Match pattern notches and maintain the specified allowance before stitching. Press the seam after sewing and finish raw edges where the fabric frays. Pressing is part of construction: it sets stitches, shapes curves, and prevents unnecessary bulk in later steps.