You have probably seen it on a product page and scrolled straight past it. 180 GSM. 220 GSM. Three letters and a number that tell you more about how a T-shirt will age than the price tag does.

GSM stands for grams per square metre — the weight of one square metre of the fabric. That is the whole definition. But that single number quietly decides whether your shirt drapes or clings, whether it survives a summer or a decade, and whether you can see your skin through it in daylight.