If the fabric is soft and the complaints point to one spot, the seam is the first suspect. Chafing usually comes from a seam with too much bulk, from a stitch whose thread surface rubs the skin, or from a seam positioned where the thigh moves against it. Ask the factory to show you the gusset construction, and compare it with the alternatives they can offer.

Review the seam type and finish

Underwear seams can be overlocked, coverstitched or flatlocked, and each feels different against skin. A raised overlocked seam inside the leg can rub during walking. Flatter seam constructions are often chosen for comfort in these areas, but they need the right machine and fabric. Ask for samples of the gusset sewn with each option the factory can produce, on your fabric.

Check where the seam sits

Seam position matters as much as seam type. If the gusset seam sits where the inner thigh moves most, moving it, or reshaping the gusset, can reduce friction. That is a pattern change, so fit-test it in several sizes, since the seam's position on the body changes with grading.

Look at leg-opening elastic and fit

If trunks ride up during walking, the gusset seam moves into the thigh. The leg-opening elastic and leg length affect this. Elastic tension can be measured with ASTM D4964 at agreed points, new and after an agreed laundering procedure such as ISO 6330. A leg elastic that relaxes after washing can let the leg ride up.

Fit-test with movement

Chafing appears during movement, so a static fitting will miss it. Ask wear-testers to walk and sit in the samples for a set period and report where they feel the seam. Keep notes on size and seam option. Then pick the construction that solves it with the least change to the look.

Write the chosen seam type, stitch, thread and gusset pattern into the tech pack, with a photo of the approved seam, so bulk matches the wear-tested sample.

Keep the wear-test notes with the approval. If chafing complaints come back on a later lot, you can check whether the seam construction or position drifted from what was tested, instead of starting the investigation from scratch.