Why the seam sits on the outside in sportswear
Short answer: Turning the seam outward removes the inner edge that otherwise lies against the skin. That reduces chafing during long and sweaty activities. The price is that the seam is more visible and wears faster, because it sits on the outside.
The edge moves away from the skin
An ordinary seam has a seam allowance on the inside of the garment. If the seam is turned, the allowance lies outside and the inside becomes completely flat. That means no edge can press on the skin as you move, and the fabric lies evenly against the body from one side to the other.
Why it makes a difference in sweat
When skin is damp, friction rises, and even a small edge can chafe within an hour. Sportswear is therefore sewn with flat or external seams. That makes sense for activities where you move for a long time and the same part of the garment rubs against the skin again and again. See bamboo sweatpants for men.
What is a seam allowance
A seam allowance is the narrow strip of fabric outside the actual seam line when two pieces of fabric are sewn together. Normally it lies hidden inside the garment and is pressed to one side. It is needed for the seam to hold, but it can be felt against the skin if it lies on the inside.
The price is looks and wear
An external seam is more visible and wears faster, because it sits on top and meets everything the garment touches. It is a trade against comfort. Some feel the external seam gives the garment a sporty look, while others prefer a calmer appearance. See bamboo long john and bamboo underpants for men.
Flat seam and external seam
A flat seam and an external seam are not quite the same. In a flat seam the two fabrics lie edge to edge and are sewn together, so the seam ends up flat. In an external seam the usual seam allowance is turned outward. Both solutions share that they remove the edge from the inside of the garment.
Is it used in ordinary underwear too
Yes, in models meant to be worn all day or under a uniform. The aim is the same as in sportswear: no edge that rubs against the skin through a long day. The seams may then sit outside or be made as flat seams, depending on how the garment is built and what it is used for.
Do external seams look cheap
That depends on the execution. In sportswear an external seam is a familiar and accepted look, and a clean, even seam looks proper. A crooked or uneven seam is more visible outside than inside and can look messy. Quality therefore shows in how even and clean the seam is sewn.
Can you turn a seam yourself
No, that requires the garment to be sewn again from scratch. A seam cannot simply be turned, because the allowance is cut and pressed in a set direction and the stitches sit in both layers. If an inner edge bothers you, try a different material or a different fit instead.
How to choose by activity
Think about how long and how hard you move in the garment. For short, calm activities the seam rarely matters. If you walk or run for a long time and get sweaty, flat or external seams are a good choice. Notice whether the seams lie where they should and whether they rub against the skin.
Frequently asked questions
Is it used in ordinary underwear too?
Yes, in models meant to be worn all day or under a uniform, where an inner edge could otherwise rub through a long day.
Do external seams look cheap?
That depends on the execution. In sportswear it is a familiar and accepted look, and a clean, even seam looks proper.
Can I turn a seam myself?
No, that requires the garment to be sewn again from scratch, because the allowance is cut and pressed in a set direction and the stitches sit in both layers.
You can find the full range at Bamboo-Clothing.com.
