As with all clothing choices, you want your underwear to be:
- Quick Drying
Most pilgrims pack two or three pair of most items of clothing and underwear is no exception. You'll be wearing a pair while the second is drying on the line at a refugio.
I highly recommend trying a few different brands and types before you head out for a month or more of walking.
You want underwear that doesn't bunch or wad--that makes for a long, uncomfortable day of walking.
One of the problems with some of the modern fabrics is that they don't breath well. This is also important. Dry skin is an imperative. If the sweat isn't wicked away from the body, you'll chafe or, worse, develop blisters or sores. These are incredibly painful in bendy places on our bodies.
Also important is that your underwear, like everything you wear, dries quickly. This is why the man-made fabrics are a great choice. I love cotton, but it stays wet and can take forever to dry. There are some rainy days and damp days when even the quick-dry fabrics take a long time.
Another advantage to many of the man-made fabrics is that they contain an odor guard. This keeps body smells to a minimum not only during your day on the trail, but also over the long haul. If odor block or odor control is a feature of the item it should be listed in the description.
All of this said, I've met several pilgrims who preferred cotton underwear. Why? Because they were heavy sweaters. The man-made fabrics didn't offer them the confidence and comfort of cotton.
This is a truth about all the lists and advice you'll encounter as you plan your pilgrimage. What works for someone else might not be the best choice for you. The only way you find out what truly will work best for you is to try it. So, buy a pair of underwear and spend six or seven hours walking or hiking. Get home and do your chores (i.e., strip out of your sweaty clothes, hand wash them in a sink or while you shower, and then hang them outside to dry). Now, put them back on one day later and see how they feel and respond.