I just picked up a six-pack of premium 100% cotton undershirts for everyday wear and was struck by how heavy they were. The six of them weighted two pounds. No wonder I carry only two or three poly/nylon quick-dry undershirts when I travel. Two pounds would amount to about one-fifth of my normal load for a trip of up to 10 days. Two synthetic shirts (which can be laundered in the sink every night or so) weigh eight ounces or less. It’s not just pants and shirts - which you can often wear two or more times without washing - that weigh a suitcase down. Carrying a daily change of cotton underclothing may make you fill good all under but may be hard on your back or shoulder. Think synthetic. But be choosy: the worst of the poly/nylon undies can be brutally uncomfortable. The good ones are a delight.