Underwear that chafes on the inner thigh

Short answer: Chafing happens where fabric meets skin in motion. Longer legs, smooth materials and flat seams solve most cases of the problem completely.

Longer legs help the most

A style with longer legs keeps fabric between your thighs instead of skin against skin. It is the simplest change, and it works for most people right away, no matter how much you move during an ordinary day at work or in your free time. See bamboo underpants for men.

Inner shorts under dresses and skirts

For dresses there are short shorts that place a smooth layer between the thighs. They sit close without squeezing, and you can wear them under almost anything without it showing, even on warm days when an extra layer would otherwise feel like too much. See bamboo inner shorts.

Moisture makes it worse

On warm days moisture increases friction, which is why the problem almost only shows up in summer. A material that moves moisture away from the skin reduces the problem noticeably, and changing halfway through the day does the rest, especially if you move a lot. See bamboo panties.

Seam placement matters more than you think

A flat seam along the leg chafes less than a raised seam, even when the fabric is otherwise the same. Look for flat sewn edges if you walk or cycle a lot during the day, since this is where the difference shows up most.

The material decides how fast it happens

A smooth, even fabric glides against the skin instead of gripping it. Rough or stiff fabrics create more resistance, and resistance turns into chafing after a few hours, no matter how well the cut otherwise fits the body.

The fit should be close but never cutting

Loose fabric shifts and creates friction, while tight fabric presses the edge into the skin and leaves red marks. The right size sits somewhere in between, and it is worth trying a size up or down before giving up on a style entirely.

Movement reveals the problem faster than sitting still

Walking, running and cycling repeat the same motion thousands of times, and this is where chafing really shows up. If you mostly sit still during the day you notice it less, but that does not mean the material is good enough for active days with lots of movement.

A cream is only a short term fix

Anti chafe cream can dull the symptom for a single day, but it does not solve the cause behind the problem. The right cut and material prevent the problem instead of you having to remember a tube every time you plan to move a lot.

Washing and wear change the fabric surface

A worn or wrongly washed fabric loses some of its smoothness over time, which makes it more likely to grip the skin. Follow the wash instructions and replace the underwear once the surface starts to feel rough to the touch.