A seam is where two pieces of fabric join, and sewing one means running thread through both layers in a straight line so they stay together
The basic motion is the same whether you use a needle and thread by hand or a sewing machine: you push the needle through both layers of fabric, pull it back out a short distance away, and repeat until you reach the end. The distance between the edge of the fabric and your line of stitches is called the seam allowance—usually half an inch on garments, though patterns will tell you what to use. Once you understand the rhythm and can keep your stitches even, you can sew almost any seam.
The difference between hand sewing and machine sewing is mainly speed and strength. A machine makes dozens of stitches per second and locks each one in place, so the seam holds under stress. Hand stitches are slower and looser, but they work fine for repairs, light fabrics, and seams that won't take much pull. Most garments and projects use a machine because the seams need to last through washing and wearing.
Key Takeaways
- A seam allowance is the distance from the fabric edge to your stitching line, usually half an inch, and your pattern will specify what to use.
- Hand sewing uses a running stitch or backstitch, with the backstitch being stronger and better for seams that will take stress.
- Machine sewing requires you to align the fabric edge with a guide on the presser foot, set the stitch length, and keep the fabric moving straight.
- Pinning the two fabric pieces together before you start keeps them from shifting and makes the seam straighter.
- Backstitching at the start and end of a seam—whether by hand or machine—locks the thread so it does not unravel.
Pinning fabric pieces so they do not shift
Before you sew, lay the two pieces of fabric with the right sides facing each other (the sides you want to show on the finished piece). Smooth them flat so there are no wrinkles or bunches. Place pins perpendicular to the edge you are about to sew, spacing them about three to four inches apart. The pins should go through both layers and stick out on both sides so you can see them.
Pins keep the two layers from sliding apart while you work. If the fabric is slippery or the seam is long, use more pins. When you reach a pin while sewing, you can either pull it out just before the needle gets there or, on a machine, sew right over it if the pin is not directly under the presser foot—though pulling it out is safer and takes only a second.
Hand sewing a seam with a backstitch
The backstitch is the strongest hand stitch for seams because each new stitch overlaps the one before it, locking them together. Thread a needle, tie a knot at the end, and bring the needle up through both layers of fabric at your starting point. Pull the thread until the knot catches on the back of the fabric.
Push the needle down through both layers about one-eighth inch ahead of where you came up. Bring it back up one-eighth inch ahead of that. Now push it back down into the hole where the last stitch ended—this is what creates the overlap and locks the stitch. Repeat this forward-and-back motion all the way along the seam. Keep your stitches the same size so the seam looks even.
When you reach the end, push the needle to the back of the fabric and make two or three small stitches in the same spot without moving forward. This locks the thread. Trim the excess thread close to the fabric. The backstitch takes longer than machine sewing but creates a seam that will not come apart under normal use.
Machine sewing a seam in a straight line
Place the pinned fabric under the presser foot (the metal piece that holds the fabric down) so the edge aligns with the seam allowance guide on the machine—usually a line marked on the throat plate, the metal surface around the needle. Most guides are marked for quarter-inch and half-inch seam allowances. Lower the presser foot to hold the fabric in place.
Set the stitch length to a medium setting—usually between 2 and 3 millimeters on most machines. A shorter stitch creates a stronger seam but takes longer; a longer stitch is faster but weaker. For most garments, medium is the standard. Hold the fabric gently with both hands and guide it straight, letting the feed dogs (the metal teeth under the presser foot) pull it through. Do not pull or push—just guide.
Start sewing by pressing the foot pedal or pressing the start button, depending on your machine. Sew forward a few stitches, then press the reverse button or lever to backstitch three or four stitches. This locks the thread at the start. Release the reverse control and sew forward to the end of the seam. When you reach the end, backstitch again for three or four stitches, then lift the presser foot and pull the fabric away from the needle. Trim the thread tails.
Keeping stitches straight and even
The most common mistake is letting the fabric angle or drift away from the guide line. Your eyes should follow the edge of the fabric as it approaches the needle, not the needle itself—the needle will do its job if the fabric stays in the right position. If you watch the needle, you will unconsciously adjust the fabric to keep it centered under the needle, which throws off your seam allowance.
For hand sewing, mark a light pencil line on the fabric before you start if you are not confident about keeping a straight path. For machine sewing, trust the guide line on the throat plate and keep the fabric edge aligned with it. If your machine does not have a clear guide, you can tape a piece of painter's tape to the throat plate at the correct distance from the needle to create a visual guide.
Removing pins and checking the seam
Once the seam is sewn, remove any remaining pins and open the fabric so the two pieces lie flat with the seam running between them. Press the seam with an iron on a low to medium heat setting, depending on your fabric type. Pressing sets the stitches and makes the seam lie flat instead of puckering.
Look at both sides of the seam. The stitches should be straight, evenly spaced, and the same distance from the edge on both sides. If one side is closer to the edge than the other, your seam allowance was uneven—this is normal when you are learning. The seam will still hold. If stitches are loose or uneven, you can pull out the thread and try again, or leave it if it is strong enough for the project.
Fixing a seam that came loose or puckered
If a seam starts to come apart after sewing, you can restitch it by hand using a backstitch along the same line, or you can use a seam ripper to remove the old stitches and sew it again on the machine. A seam ripper is a small tool with a curved blade that slides under stitches and cuts them without damaging the fabric. Slide it under every third or fourth stitch and cut, then pull the thread away.
Puckering—where the seam bunches or wrinkles—usually happens when fabric is pulled too hard while sewing or when the presser foot pressure is too high. If you notice puckering, remove the seam and try again with a lighter touch. On a machine, you can also lower the presser foot pressure if your machine has that adjustment. Pressing the finished seam with an iron often removes minor puckering.
Frequently Asked Questions
What is the difference between a seam allowance and the stitch line?
The stitch line is where your needle actually goes through the fabric. The seam allowance is the distance between the stitch line and the edge of the fabric. A half-inch seam allowance means there is half an inch of fabric between the edge and where you sew. Your pattern tells you what seam allowance to use.
Can I sew a seam without pinning first?
You can, but pinning makes it much easier to keep the seam straight, especially when you are learning. Fabric can shift and slide, and pins hold the two layers together so they stay aligned. For small, straightforward seams on stable fabric, pinning is optional, but it takes only a minute and prevents mistakes.
Why does my machine seam come apart after washing?
The most common cause is backstitching too short or not at all—the thread unravels from the ends. Always backstitch three to four stitches at the start and end. If the seam comes apart in the middle, the stitch length may be too long, or the thread tension may be wrong. Check your machine manual for tension adjustment.
How tight should I pull the thread when hand sewing?
Pull the thread snug enough that the stitch holds the fabric together without gaps, but not so tight that it puckers the fabric or breaks the thread. The fabric should lie flat on both sides of the seam. If you pull too hard, the fabric will bunch; if you pull too loose, the stitch will not hold.
What stitch should I use for a repair seam on an existing garment?
A backstitch by hand is the strongest choice for repairs because it locks each stitch and will not unravel if one thread breaks. If you use a machine, use the same stitch length as the original seam so it matches. For a seam that is splitting, you can sew along the inside of the original stitch line to reinforce it without removing the old stitches.