Ask most brands what a shirt is made of and you will get “100% cotton”. It is true and it is nearly useless. Cotton spans a voile you can read a newspaper through and a duck canvas you could make a bag from. The fibre tells you almost nothing about how the garment will behave.
The number that does is GSM: grams per square metre. It is the mass of a square metre of the finished cloth, and it predicts drape, opacity, warmth and how the garment will age better than anything else on the page.
The ranges that matter
For shirting, the useful bands are narrow. Below about 120 GSM you are in voile and fine poplin: beautiful in June in Chennai, translucent under an office light, and prone to blowing out at the elbow within two summers. Between 130 and 180 is the everyday range, where most oxford and poplin sits. From 200 up you are into twill and flannel, cloth with enough body to hold a collar roll and to work as a light layer over a tee.
Our olive overshirt is 280 GSM. That is heavy for something called a shirt and light for something called a jacket, which is precisely the point. At 280 the twill has enough weight to hang straight from the shoulder without collapsing, and enough give that it does not feel like outerwear indoors.
Why weight is under-reported
Partly because it is unglamorous. Mostly because it is a commitment. A GSM figure is checkable: put the garment on a scale, measure it, do the arithmetic. A brand that publishes 280 and delivers 240 has made a claim someone can disprove. “Premium cotton” cannot be disproved because it does not mean anything.
The trade-offs it does not capture
Weight is not everything. Two 180 GSM cottons can feel entirely different depending on yarn count, weave and finish. A high-count poplin at 180 is smooth and slightly cool; a slubby chambray at the same weight is textured and matte. Weight tells you the category; the weave tells you the character.
What weight does reliably tell you is whether the garment will do the job you are buying it for. If you want something to layer under a knit, do not buy 280 GSM. If you want something to wear open over a tee in November, do not buy 120.
All of it is on every product page here, along with the mill and the tested shrinkage. Not because it is marketing, but because it is the information you would ask for if you were standing in a shop holding the thing.